Off Road Modifications - Intermediate

In this section we will deal with slightly more advanced modifications to off road vehicles, they will not be in any particular order asthey may well be interconnected to a range of other modifications being considered for the future. It is important to consider this interconnection of systems when planning a range of vehicle upgrades as many will influence other modifications, and this is important when planning upgrades, and why a plan isnecessary.

It is up to those intending to modify their vehicles to consider the range of modifications, and proceed accordingly.


Engine electrical upgrades are popular for many reasons, it may be that you're considering fitting a high output item such as a winch, or have already fitted one and your electrical system is too small to operate it,or you use lights or other items on the vehicle for long periods with a stationery engine.

To proceed we need a simple understanding of vehicle electrical systems, we all know they are 12 volt systems, but we need to know how to calculate the various terminologies such as volts, amps, and watts into one common formula. This is simply because we only use volts and amps when we calculate our electrical systems requirements.

Our Formula is:

Volts Times Amps =Watts

Watts divided by Volts =Amps

Watts divided by Amps =Volts

Using these basic formula we can calculate any electrical load on a vehicle, we can also use this to calculate our requirements for specific upgrades and ensure we have the right output alternator and battery for our needs.

Electrical loads vary from vehicle to vehicle, it isimportant that we appreciate that there are two types of electrical loads, these are defined as constant loads and intermittent loads.

Constant loads are the loads which are regularly used for long periods; these may include lighting for night time driving, the constant engine requirements which would include diesel solenoids and engine management systems which operate while the vehicle is running. In simple terms these are long term electrical requirements

Intermittent loads are those electrical loads which may be high, but for a short period, or periods of time; these would normally include glow plugs, starter motors, indicators, and rear screen and vehicle heaters/demisters.

As an indication of a constant load, a Fourtrak will normally require about 5 amps just to run the engine, this assumes every other electrical load is switched off, so we already have a power requirement before we begin calculating, just to run the engine.

Now we need to translate the various measurements into one standard measurement to allow us to calculate our power requirement for a standard vehicle, basically we will be working out the power requirement for a vehicle under the best and worst operating conditions.

Best Conditions

Engine running 5Amps

Indicators 21+21+5+3Watts

This breaks down to 21 watts, front indicator bulb, 21 watts rear indicator bulb, 5 watts wing repeater lamp, 3 watts dashboard indicator bulb which = 50 watts.

Using our formula we see Watts divided by Volts = Amps or

50 divided by 12 = 4.166 Amps,we will round this up to 5 amps to allow for the power consumed by the flasher relay.

Combine this with the engine running then the vehicle isusing 10 amps with just the engine and indicators operating, we have a 5 amp constant load and a 5 amp intermittent loading.

Worst Conditions

Constant Loads

Engine running 5Amps

DippedHeadlights dipped 55+55+5+5+5+5+5Watts (2 head + sidelights + No plate light)

Front Fog lights 55+55+3Watts

Dashboard Lights 5+5+5+5Watts

Rear Fog lights 21+3Watts (assuming only one is fitted – double for 2)

Heater 10 Amps

Intermittent Loads

Rear Screen Heater 30 Amps

Indicators 5 Amps

If we add up our total constant loads:

55+55+5+5+5+5+5+55+55+3+5+5+5+5+21+3 Watts =287 divided by 12 = 23.91 amps. Add on our engine running andheater amps we have a round figure of 39 amps for our constant loads.

If we add our intermittent loads to this we have a total power consumption of 74 amps; 39+30+5 Amps.

If we have an alternator of 70 amps capacity, we see that it is already exceeding its rated output, unless the engine is running the alternator at its maximum output speed, its output will be less than its maximum 70 amps. This means the additional power is supplied by the battery which acts as a reservoir; it will supply additional power requirements for short bursts over short durations so everything requiring power is supplied with power.

When we do not use the indicators it saves us 5 amps, so if the alternator is operating at its maximum output it can now cope with the load of 69 amps and supply the battery with a charging current of 1 amp. If we switch off the rear screen heater after 5 minutes we reduce our power consumption by another 30amps, this means if the alternator is effectively now recharging the depleted battery by supplying a charging current of 31 amps.

Fluctuations do occur, many will notice the windscreen wipers have not been included, this is deliberate to highlight this point of electrical load fluctuation, if they are used it could add another 3-8 amps of load, or reduce this load if they are not used. Windscreen wipers are one of those items which can be either a constant load in heavy rain, or an intermittent load where they are used for one sweep of the windscreen in very light mist or drizzle conditions.


Batteries are misleading for many people, they have several ratings, and one very misleading rating; the general ratings are CCA, cold cranking amps, and AMP/HOUR

Ratings, which will be explained.

CCA is the amount of current which will be supplied for the starter motor, the higher the CCA rating, the lower the load will be on the battery when the starter is used, the CCA rating is the current which it will produce for one minute. This measurement is used by most battery manufacturers, but not all, and is applicable for other heavy loads such as winches as these require a minimum CCA battery to operate effectively. Too low a CCA, the winch operating time and pulling capacity will be considerably reduced, this is because when such high loadings are imposed on a battery it is effectively being short circuited for these short periods of operation.

A/H ratings are a misnomer, theoretically a 100 A/H rated battery will supply 100 amps for one hour, but in reality it will supply 100amps for about 10 minutes only, so why is this? It is because most batteries are rated on the 20 hour rating system. This means that it will supply 100 amps, but over 20 hours, in reality it will supply 100 divided by 20 which equals 5amps; we can now say the 100 A/H battery supplies 5 amps constantly over 20hours. If this 5 amps is exceeded it will deplete the battery much more rapidly, and it will not last anywhere its designated A/H rating.

We now have the useful knowledge to work out our electrical loadings, and enough knowledge to make informed choices to select electrical equipment, and how to use our electrical power requirements in designing additional circuits.


Split charging circuits are often overlooked, and often create additional problems for modified vehicles, winches create most problems, but others who use vehicles as working vehicles with additional lighting also suffer from flat batteries. This may be because they need additional work lights fitted, but do not leave the engine running to keep the battery charged as it costs in fuel, or those who use vehicles as expedition or camping vehicles running additional items such as fridges.

Split charging is a method of using one alternator to charge the vehicles main battery, and an additional battery/s to supply other loads, independently of the main vehicle battery; this retains the vehicle battery at full charge to start and operate the vehicle. These systems are already used by caravans and certain trailers which have integrated battery operated systems such as electrical tipping, or integrated electrical winches, so vehicles may already be fitted with them.

Basic split charging systems operate using two batteries, the main battery powers the main vehicle functions as normal on the vehicle, the additional battery powers any accessories, alternator charge is divided between the two batteries with the engine running. This is done in two ways, the basic split charging systems use a special relay to bias the charge to the main vehicle battery as this is the predominant battery, and the most essential as it starts and runs the vehicle, the spare charging capacity is then diverted to the auxiliary battery to charge it. This ensures the main battery remains independent of the auxiliary battery, and the auxiliary battery may become totally discharged through use, but our main vehicle battery remains fully charged so the vehicle can start and run.

These basic split charging systems are fine for simplicity, but where a large auxiliary battery is used it may take a long time to recharge with these basic systems, and they are best used where the auxiliary battery is of a smaller capacity then the vehicle battery. Or, if an auxiliary battery larger than the vehicles main battery is fitted, but can have a long charging period, such as a caravan being towed for many hours to a holiday destination. This limits their use if an auxiliary battery which is larger than the main vehicle battery is used, which is common when electric winches are used, particularly if the winch CCA are higher than the rating of the main vehicle battery. This means overly long recharging times are necessary to recharge the auxiliary battery to allow winching to commence if the winching operation is long, or the winch is running near its maximum pulling capacity. Where winching operations are necessary, batteries need a faster recharging time to allow them to recover quickly so winching can continue, not long recharging times which are possible with our towed caravan.

Newer split charging systems utilise modern electronics, many are programmable to allow the bias to be maintained, or altered between two or even more batteries, have inbuilt monitoring to constantly monitor the main vehicle, and auxiliary batteries. These allow this bias to be pre programmed between two or more modes, and are more efficient in applying this variable bias with no detrimental effects to the main vehicle battery, most incorporate a simple panel to show the charge status of the two or more batteries. Such display panels often allow the status of the main vehicles battery to be monitored while the bias is temporarily altered to provide all the alternator charge to the auxiliary for a short period of time, recharging the auxiliary battery quickly. If the main vehicle battery becomes depleted to too low a level it will show on the display panel, many now incorporate a visual and audible display of such a condition and allow the modes to be switched back to a maintenance charge of the main vehicle battery. Many already have an inbuilt, or programmable automatic switching facility to bias a proportion of charge back to the main vehicle battery, so are virtually foolproof if correctly programmed.

In real terms they maintain the vehicle battery to its operating condition, but divert considerably more charge to the auxiliary battery, this recharges it quickly, but prevents overcharging of the auxiliary battery. Flexibility is its main advantage, it allows a much larger auxiliary battery to be fitted, considerably larger then the main vehicle battery, and has no problems in providing a maintenance charge to the vehicle battery, but biasing the bulk of the charge to the auxiliary battery. This does what it says, maintains the vehicles main battery to a minimum state of charge, but diverts the bulk of the charge to the auxiliary battery for quicker charging, and shorter down periods between winching operations.

Many new systems have an emergency back up system, this means it allows the main vehicle battery to be maintained to a minimum state of charge which will be monitored automatically by the system and stored in its memory. It will automatically sense a rapid discharge of the auxiliary battery and switch the main charging requirement to the auxiliary battery, it will also sense the load used by the main battery and supply this to it automatically.


We are using an electric winch for an arduous winching operation for a long period, the vehicle is running at a fast tick over speed to supply power, at this speed our now uprated alternator is supplying 80 amps instead of its 120 amps, due to the lower engine speed.

Our vehicle is using no electrical loads other than that required to run the engine, 5 amps, we have our heater on to remove heat from a stationary vehicle and keep the interior warm, this is on a medium setting and uses another 7 amps. Our total power consumption from the main battery is now 12 amps, the split charger will sense this 12 amp load and supply this directly to the vehicles main battery, thus replacing the charge it uses, but also adds a small percentage which is normally 10%. This now means our auto sensing system is actually sending 5A+7A+10% back to the main battery, which in real terms means the split charging system sends a charging current of 13.2 amps to the main battery. This leaves us with 66.8 amps output, this is sent directly to the auxiliary battery to recharge it quickly to allow us to commence winching as quickly as possible.

Having a monitoring system and display panel with out system means the operator can instantly see the state of charge of both batteries.

Such advantages can be practically demonstrated, in a recent example I fitted an electric winch to a vehicle, its main vehicle battery was rated at 550CCA, and 85A/H, and winch specifications were a minimum CCA of 670A/H. This meant the winch would not work efficiently with the standard vehicle battery, it would destroy this battery in a short time and not pull anywhere near its maximum rating; effectively it was useless in all aspects. Further checks of the winch specifications revealed an optimum battery size of 730-900CCA for maximum winch efficiency, a digital split charging system was chosen along with an Exide battery rated at 880 CCA. This was fitted to the vehicle and all was well, the winch worked perfectly as the auxiliary battery was well within the winch manufacturer’s specifications, and it prevented the vehicles main battery having its life considerably shortened. This has practical and cost implications, maintaining the original vehicle specification battery means a wide range of original specification batteries are available when the battery requires replacing, such choices mean many makes and price ranges are available.

Here we highlight another problem, battery life; if the main vehicle is of too small a capacity to power the winch it will have its working life considerably reduced, it will also invalidate its warranty, and the warranty of the winch.

One additional consideration is that modern diesel engines, and all petrol engines use engine management systems, if the main vehicle battery becomes discharged it will not provide enough electrical power to maintain these management systems and the vehicle will cut out. If it cuts out due to battery discharge, it will not restart, this is not a problem if there is another running vehicle and jump leads available, but for someone on their own it can become a major problem.

Many digital split charging systems are now available, and at reasonable prices; so if you have additional heavy loads such as an electric winch, additional high powered lights, work lights, or fridges, always ensure a split charging system is a high priority. These systems are constantly developing, and at an alarming rate, so always check the different systems for useful features, not worthless gimmicks, and ensure you get a system which suits your requirements.


HHaving read and understood the electrical requirements and loads on an alternator, and decided upon a choice of electrical upgrades for our vehicle, and the additional loads they impose, we have worked out that our original 70 A/H alternator is not sufficient. We have formulated a planned number of upgrades which include a split charging system, winch, additional lighting, etc; and calculated their additional electrical loadings, and decided that a more realistic sized alternator to meet these additional requirements is necessary.

Many other factors enter the equation before we make a decision on what size and type of uprated alternator we require, mainly this is the type or sort of vehicle use which influence the decisions we are about to make. Is the vehicle subject to a lot of mud or wading, if so there is a real risk of the alternator flooding or suffering from abrasion damage from this abrasive material entering the alternator, its actual position also becomes more critical.

Many engines in a number of vehicles are “generic engines” these are an engine which may be used in a number of different vehicles across many manufacturers ranges, or a shared engine between many manufacturers. Many are simply a compromise with the alternator mounted fairly low on the engine, this is not beneficial to those whose vehicle is used in deep mud or water as this will quickly fill the alternator and destroy it.

This leaves us with a few choices, can we fit an uprated alternator to our engine, and what type of alternator will we fit, and where are we going to mount it? All important considerations before we make our final choices. Many people will find it possible to mount the alternator higher on the engine, and have the facilities, or access to the expertise to mount a different alternator, this leaves us with which type to choose. If we look at the now common 120-180 amp alternators which are popular with newer vehicles, we see we have the variety of outputs to choose from at reasonable prices new, or available from vehicle breakers, second hand, at reasonable prices.

We also need to look at the alternator specifications because they all have a maximum running speed, this is the actual alternator shaft running speed, not the engine running speed, and petrol engines run at higher speeds than diesel engines. If we look at the pulleys of the crankshaft and the alternator pulley we see they are different sizes, this is effectively the gearing which allows the alternator to run at its optimum speed without overspeeding, and provides its maximum power output.

Many people make this fundamental mistake, they fit an uprated alternator but do not consider the gearing of the alternator, in many cases the alternator shaft speed is too low and its output is less than the original, correctly geared alternator. This is most common where an alternator from a petrol engine is fitted to a diesel engine with its much lower engine speed, and the engine cannot drive the alternator at sufficiently high speeds due to its incorrect gearing. Such fine tuning is critical when swapping alternators. Most alternators run at a maximum shaft speed of 8000 RPM, some are higher, others are lower, this is why the alternator shaft speed is so important, it needs to be correctly geared to near its maximum running speed to obtain its maximum output. This is why this information is critical, such information is commonly available from reconditioning experts, auto electricians, and the manufacturers themselves, as are a variety of different sized pulleys to suit the alternators.

Now we have this information we can make an informed decision, our vehicle spends a lot of time wading through deep water and mud, we can get an uprated alternator of around 120 amps, and it can be mounted higher on the engine. Our selected alternator is available cheaply new or second hand, and it has a variety of pulleys available to allow us to fine tune it to its optimum operating speeds, all we have to do is obtain and fit it.

What if we cannot move our alternator to a higher position on the engine? It is fitted with a number of other systems, but we do a lot of wading in deep water and need something which will not be damaged from flooding. The answer is simple, fit a marine alternator, these are waterproof and come in waterproof closed types, or waterproof open types, they are more expensive than automotive alternators due to their limited applications and low sales, but come in a variety of high outputs. If we consider the cost we see they normally cost twice the price of an automotive alternator, but if we blow an automotive alternator and have to replace it, it works out the same in the costs, but marine alternators are far more durable than automotive alternators. In real terms it means that they last far longer anyway, and are designed for harsh working environments, so often work out far cheaper in the long term, so are worthy of consideration when uprating alternators.

Other factors have to be considered, many car derived alternators come with a single pulley, or a different pulley to our vehicle, these may be the poly-V type drive belts, where as our vehicle may be fitted to accept V belts. This is where we need to change the pulleys to accept our vehicles system of drive belt.

Twin alternator drive belts are necessary for most modified 4X4’s as they are often working in deep mud and water, this gives additional grip or traction on a wet or muddy crankshaft pulley as it is low down on the vehicle. Most marine alternators come with twin pulley’s, some come with as many as four drive belt pulleys.

Alternator wiring is crucial, if we suddenly jump from a 70 amp to a 120 amp alternator the vehicle wiring will need upgrading, vehicles wiring is again a compromise, and only just suitable for the original alternator. Always fit larger heavy duty cabling rated higher than the alternator rating, this reduces electrical resistance and eliminates many potential problems in the future, always fit sealed rubber bots to the alternator and other connections to protect them from water and dirt ingress. Such contamination creates corrosion at these electrical connections, which generates problems in years to come.

Alternators also have two sensing methods to determine their outputs, these are called machine sensing and battery sensing types, most automotive types are machine sensing as they are the cheapest, and sufficient for most automotive applications. Machine sensing types are where the alternator itself determines and regulates its output, but it does not compensate in its load regulation for any connections which deteriorate over time, and may become corroded; they are simple and cheap.

Battery sensed types have one additional length of wire with no connections which runs from the battery, back to the alternator, this measures power at the battery to regulate the alternator output. Benefits include the ability to increase the alternator output to compensate for any deteriorating or corroding joints in the wiring, and are the best type to have they charge the battery to much nearer its optimum. Most automotive alternators can be converted from machine sensing to battery sensing types; it is simple as soldering an additional connection internally within the alternator and providing an additional external connection. Most automotive electricians will do this for a few pounds.

Now we have decided to fit an uprated alternator and a split charging system we have to give a little consideration to our wiring, this is where we revert back to our original plan, and why it is important to have a plan.

When fitting any high load consuming equipment such as winches, always fit a battery isolation switch, these allow us to switch off the load when it is not used, if a winch becomes wet it may actually short circuit slightly and discharge the auxiliary battery. If a wiring fault occurs it could actually set fire to the vehicle and burn it out, so always switch off high electrical loads until they are used, it also prevents people from tampering with them; and yes this is not uncommon.

When installing wiring for auxiliary circuits always use rubber grommets where cables pass through vehicles bodywork, this prevents chafing and short circuits in the wiring, and use one of the many systems to secure cabling to the vehicle.

Auxiliary circuits should always be correctly designed and installed, this often means fitting an additional fuse box and a relay holders, never skimp on these items as they are there to protect, and are cheaply and readily available. My recommendation is that any accessories should be wired on their own circuits, and separate from the main vehicle wiring, this leaves the vehicle wiring as standard, and eliminates many of the bodged wiring problems we all see. Using many of the current wiring accessories, we can make these accessory circuits a different colour for easy identification, this means troubleshooting is much easier should we develop a fault, and it does not affect the normal vehicle electrical functions.

Always make a circuit diagram and keep it, computers make these easy to store, and a hard copy in the vehicle is usually beneficial for circuit identification, such as identifying a blown fuse or a defective relay. Note the wire colours for individual circuits, the type (pin configuration) and ratings for relays, and where possible, the part number for replacements.

I personally use a company called “Vehicle Wiring Products