M201 Batteries

The batteries specified for the M201 24 volt are two 45 amp-hour type. The un-rebuilt jeep in the photo opposite is M201 no. 25874 with everything under the bonnet pretty much as it left the factory apart from the liberal waxing it received before going into storage at some point. Batteries were removed from jeeps to be sold at auction ( a French regulation) and are obviously not the original item but are ex-French army NOS which bizarrely were actually sold at auction.

The batteries shown are the 45 amp-hour size made for the army by VARTA in Germany.

 Note also the quick release connectors used.

 

An important point to make at this stage is that the batteries should always be replaced as a pair with the same capacity and from the same manufacturer otherwise charging will not be equal and battery life will be reduced.

My personal quest for authenticity with my jeeps does not, however, run to batteries! The main reasons are the cost of 45 amp-hour batteries, two at a time, together with the fact that lack of daily use during the summer and a long period of inactive storage over the winter is not good in terms of battery life expectancy. The principal culprit is something called 'sulphating', the build-up of a layer of lead sulphate on the negative plates that occurs where a lead-acid battery is not kept fully charged.  This reduces the capacity of the battery to hold charge, increases the internal resistance and ultimately the battery cannot deliver the 'umph' required to start the engine however long you charge it for. I use the cheapest batteries I can get that will fit, usually 038 type intended for Rover Minis and the like. They are a bit small for the frames that hold them down but with a bit of packing all is fine and unless you are using a radio transmitter there is little if any need for 45 amp hour capacity.

To measure the amount of charge in a lead-acid battery accurately really requires measuring the specific gravity of the electrolyte using a hydrometer. These are still available though of course most modern batteries are 'sealed for life'. In any case using a hygrometer is a tricky business which can lead to acid getting on things it shouldn't. The voltage of a lead-acid battery can be used as a pretty good guide to the level of charge provided it has been idle for at least 8 hours with no load attached (isolator off). (note the table is for one 12 volt battery)

The evil process of sulphating begins to occur whenever the battery voltage falls below 12.4 volts or about 70% charge. The rate of sulphation increases dramatically with further falling charge level. The simplest and cheapest way of keeping it to a minimum is to ensure that the batteries are kept fully charged as possible which is particularly important if the jeep is laid up over the winter period. There are anti-sulphating additives available and special electronic chargers that produce a pulsed charge that can even reverse the effects of sulphation but keeping a full charge is the simplest and most cost-effective.

 
BCI Standard figures
CHARGE VOLTAGE
100% 12.65
75% 12.45
50% 12.24
20% 12.06
FLAT 11.89

N.B. these readings are for standard batteries. 'Calcium' type batteries will be 5 - 8% higher

Unlike a 'leisure battery', an automotive battery is not designed for what is called 'deep cycling'. It can withstand the occasional drop into the yellow zone provided it isn't left there for long but allowing it to go flat before recharging will reduce its life significantly and leaving it flat for some time may even end its operational life.

REGULATOR BOX

When the two 12 volt lead acid batteries in an M201 are being charged the voltage rises to between 28 and 29 volts, any higher than this and the batteries will be seriously damaged. One function of the regulator is to ensure that this does not happen, its full function is described on the regulator page. The exact setting is a compromise and is dependent on the temperature range that the jeep is likely to have to operate in. The factory setting was a nominal 28 volts, perhaps not surprising given the need for desert operational capability.

VOLTAGE LIMIT

28 volts

29 volts

ADVANTAGES Maximum service life; the battery remains cool during charge; the vehicle can operate in temperatures exceeding 30C Faster charge times and the battery is less likely to suffer from sulphation.
DISADVANTAGES Slow rate of charge and sulphation of the negative plates will occur if the battery is not regularly given a top-up charge. Battery will be seriously over charged at temperatures >30C and 'wear' of the positive plates (known as corrosion) is increased.

GOOD PRACTICE TO PROLONG BATTERY LIFE

  • Replace batteries as a matched pair and avoid using one battery to power 12 volt accessories, camping lights etc.
  • Check the state of charge regularly and charge the batteries if this falls below 75%. This is particularly important if the jeep is laid up for the winter.
  • If the battery does go flat then charge it up as soon as possible. The run home from a show is unlikely to be far enough to fully recharge the batteries.
  • Check the voltage regulation - it should be between 27.5 and 28.5 volts. If it is outside this range the regulator certainly needs adjusting.

FINAL NOTES

Bubbling or 'gassing' becomes quite vigorous as the battery nears full charge if you are using a traditional type of battery charger. It requires a period of time like this for the batteries to actually reach 100% charge.

Most batteries these days are classed 'low maintenance' and do not require 'topping up'. When they do it is usually a sign that they are pretty much time expired. If your battery is fairly new and requires this then the chances are that the batteries and being over-charged and the regulator requires attention.

A fairly new battery put away for the winter fully charged should hold its charge till the next season but as a battery gets older it will tend to slowly self-discharge and will need a top-up charge from time to time.

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