How to Lighten Car Alternator Load


Postby giamar339 » Thu Jun 07, 2018 1:25 pm

Hello to all
I would like to know how can I lighten a car alternator load.
I am trying to charge 3 car battery of 12 volt 45 amps with an alternator of 12 volt 45 amp in my garage.
The alternator run with a small petrol motor but it became "heavy" if I connect more battery to it.
I know there is a cheap way to make it lighter but I don't remember where I have read it.
Thank you very much
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Postby ericmark » Fri Jun 08, 2018 12:39 am

Assuming old alternator not one which connects to engine management then you need to reduce field current, so a resistor between one of the brush wires, easy way is a bulb holder and you swap bulb to higher or lower wattage to increase or decrease max output.
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Postby Mr White » Fri Jun 08, 2018 4:39 am

Never connect batteries in parallel if you want to charge them, the one that is most discharged will dictate how the others are charged and consequently they will be over charged.
Also using a petrol engine to run an alternator to (Try) to charge batteries is an expensive waste of time and petrol, use a battery charger.
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Postby ericmark » Fri Jun 08, 2018 8:27 am

There are many ways to control the charging of lead acid batteries, the simple method is voltage, we use for a 12 volt battery between 13.2 and 14.8 volt (OK an old two bobbin regulator was set at 16 volt open circuit voltage, but connected it never reached 16 volt.) At around 13.8 volt you can connect a whole series of batteries in parallel with no problem. However with stage chargers which could charge at 14.8 volt then yes each battery should be independently charged.

With a stage charger often the charge current as well as voltage is measured, so typical is.
1) Max output until volts reach 14.8.
2) Volts held at 14.8 until current drops to 5 amp, exact amp limit depends on amphour rate of battery.
3) Volts held at 13.4 volt to maintain the battery until connection lost.

You can get stage charger regulators for alternators, Sterling make some, but they are normally only used with boats.

You can also get battery to battery, and alternator to battery inverters, again mainly boats, it will allow for example two 80A alternators to combine outputs and charge 3 x 160 Ah batteries in parallel using pulse charging very fast, and the forth engine start battery is just float charged at 13.6 volt.

However in the main vehicle alternators from old cars i.e. not connected to engine management, use a simple voltage regulator, some tractors have a temperature sensor under the battery, but in the main you could have 20 batteries in parallel using a vehicle alternator without a problem.

What was often done was to fit relays, so batteries only connected in parallel during charging, but separated when being used. The typical split charging relay so popular years ago with caravans.

On the Falklands we ran out of regulators for alternators, so to keep the fleet running we fitted a tail lamp where the regulator should have been, and swapped bulb sizes to get charge rate some where near to what was required.

So the same method could be used to de-rate the alternators, a simple resistor between the supply to field, i.e. the brushes.

Yes there were some specials like the CAV 203 alternator where it was current regulated, but really these were only used on buses, unlikely to find on standard car or wagon.

The best way to regulate the output would be speed, so larger pulley on alternator (use a dynamo pulley) will slow down alternator so allowing petrol engine to speed up and produce more torque, or of course smaller pulley on petrol engine.
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