Parallel charging of batteries - Help please

[Barry]

Hi all,
I need help with a circuit so that I can charge two lead acid batteries on my car via the alternator BUT current is only drawn from the main battery which always gives me a fully charged battery on standby.
Being a Newbie I thought of maybe adding diodes between the two batteries but what would prevent the second battery then from overcharging?
To solve this thought of possibly putting a circuit after the first battery and after diode with a relay and some type of voltage sensor to take the second battery out of charging circuit when fully charged.
Ideas and advice as always much appreciated.
Thanks.
Barry.
PS The solution is probably quite easy but I cannot see it!

 

[Nigel Goodwin]

It's not simple, but it IS commonplace, try asking at a caravan supplies shop!.

 

[zevon8]

The simplest form of battery isolator used in motor homes and such with 2 batteries is nothing more than 2 large diodes mounted in a large heatsink. The 2 diodes have their anodes conected via metal bar, with a bolt on the metal bar connecting to the alternator. Each battery is connected to one of the cathodes of either diode. This allows the alternator to charge both batteries, but the diodes prevent the loads on the "auxillary " battery from discharging the " cranking " battery.

The only problem with this system is that some alternators rely on the battery to "excite" the field winding on the alternator, and with the diode in the circuit this won't happen. Some Delco alternators are like this. Easiest way to tell if the alternator requires system power to function is if there are more than one secondary power connection to the alternator, besides a " charge" connection for an idiot light or ammeter. Isolators are made to get around this problem, they are only slightly more complictated.

These devices are actually quite cheap to buy when you take into account the price of two 100 Amp diodes, a large heatsink, and the fun of building one. The diodes need to be rated for atleast 50% more than the full load rating of the alternator, to account for the ambient temperature under the hood of a vehicle, where the device is likely to be installed.

 

[Barry]

Hello all,
Thank you for your input this far.
Really appreciated.
Barry.

 

[Klaus]

The problem with using diodes in a charging circuit is that diodes have a voltage drop when conducting. So, unless the alternator senses the charging voltage directly at the battery, there is a very good chance the battery gets chronically undercharged.
There are commercial battery isolators available that supposedly overcome this, I would NOT recommend using diodes without remote charge sensing.
My car has a dual bettery set up. It simply uses a high current relay to connect both batteries together when the engine is running. With the engine stopped the auxiliary batteriy is isolated from the main battery.
Klaus

The simplest form of battery isolator used in motor homes and such with 2 batteries is nothing more than 2 large diodes mounted in a large heatsink. The 2 diodes have their anodes conected via metal bar, with a bolt on the metal bar connecting to the alternator. Each battery is connected to one of the cathodes of either diode. This allows the alternator to charge both batteries, but the diodes prevent the loads on the "auxillary " battery from discharging the " cranking " battery.

The only problem with this system is that some alternators rely on the battery to "excite" the field winding on the alternator, and with the diode in the circuit this won't happen. Some Delco alternators are like this. Easiest way to tell if the alternator requires system power to function is if there are more than one secondary power connection to the alternator, besides a " charge" connection for an idiot light or ammeter. Isolators are made to get around this problem, they are only slightly more complictated.

These devices are actually quite cheap to buy when you take into account the price of two 100 Amp diodes, a large heatsink, and the fun of building one. The diodes need to be rated for atleast 50% more than the full load rating of the alternator, to account for the ambient temperature under the hood of a vehicle, where the device is likely to be installed.

 

[bryan1]

One of the first electronic projects I did was a battery isolator. Siliconchip did the project ages ago and I just bought the kit from Jaycar. Anyway it's a complex kit to make up for a beginner but the results spoke for themselves. I put the unit in my landcruiser and the auxuliary battery was used to keep the beer cold in a snapon esky (peltier based). I never had a flat main battery and after making another kit to cut the fridge out at 11 volts the second battery didn't discharge too low. I talked with the bloke I sold the ute to after 2 years and he said it was still working perfect. So rather than re-invent the wheel checkout that kit ( search thru the siliconchip archive will find it) and make it up. To buy a commercial equivelent would set you back a couple of hundred dollars.

Cheers Bryan

 

[zevon8]

I agree with what both Klaus and bryan1 have said, It is always best to monitor the battery system directly. This is what we do at work, both ameter and state of charge guages are used. Often we add a " low battery alarm" in vehicles that will operate from the auxillary bank for long periods.

The 2 diode system works well enough for short term auxillary use, if regular maitenance of the battery system is followed. The system can be improved using high current FET's with voltage sensing for charge and drop-out, but as mentioned above, there is no need to re-invent the wheel if you want a more complicated or accurate system.

My main point was that the garden-variety isolators are often just the 2 diode system.