Can Different Same-Voltage Batteries Be Used in Parallel for Discharge Only?

[diy50per]

Maybe somebody can help me with the following question on batteries: I
have several batteries which I have to charge from different sources.
I have solar panels on the roof of my RV, 2 by 3 meters. On a sunny
day, they can charge maybe the equivalent of a 100 ah battery. But if
I run appliances during the day, or it's cloudy, the charge from the
solar panels does not carry me through the night. So I bring reserve
electricity with me in the form of several batteries that I fully
charged at home. I understand that it's unwise to combine different
batteries in a bank because of different charging parameters. But I
wonder whether different batteries of the same voltage could be used
in a parallel circuit for discharge cycles only?
What I would do is this: disconnect the solar controller in the
evening, then do a parallel circuit of the half charged battery
usually connected to the solar panels, plus 2 batteries at different
charge levels remaining from charging at home. Then I would have
enough electricity through the night, and an additional rationale
would be that none of the batteries get fully discharged.

 

[DesmondD]

Hi, I'm an auto electrician. Your idea would work. if the batteries are at different levels of charge they may make a small spark when you connect them but they will simply equalize.
With my own camping van I try to use as little electricity as possible - use gas for any form of cooking or heating and use low current lighting and a laptop for watching movies :0
Hope that helps.

 

[diy50per]

Thanks a lot! I am trying it right now.

 

[DesmondD]

Thanks a lot! I am trying it right now.

What kind of charging do you have while driving? The alternator is very powerful and a good source of a quick charge-up. I have a VSR [voltage sensitive relay] hooked up to mine as well. When the engine starts and the voltage climbs above 13.2v it connects my 'house battery' to the vehicle's battery/charging system which allows about 80 amps of charge from the alternator.
After the ignition is turned off and the voltage drops the relay disconnects them so that the vehicle battery can't be run down.
My original photovan just had a normal manual isolator switch that I had to remember to turn on and off.

 

[CWatters]

The basic question you are asking is "Can batteries be connected in parallel". That depends on their chemistry. Different types of battery (eg Lead Acid, NiCad, NiMH, Lithium) do have different charging requirements and it would be very unwise to mix them regardless of the voltage. However if you are talking about lead acid cells only then sure that can work.

Perhaps bear in mind that they will tend to balance out. So suppose you start with two 100AH batteries one nearly flat battery and one fully charged. You connect them in parallel and watch TV for awhile. Next day when you disconnect them for charging you might find you have two batteries both slightly under half full (say 2 x 40AH). If it's bright and sunny that day then you may be able to generate 100AH but only have space to store 60AH...unless you monitor the battery and swap it over so both are charged.

A solution that charges and discharges both batteries in parallel might be better.

 

[DesmondD]

Perhaps bear in mind that they will tend to balance out. So suppose you start with two 100AH batteries one nearly flat battery and one fully charged. You connect them in parallel and watch TV for awhile. Next day when you disconnect them for charging you might find you have two batteries both slightly under half full (say 2 x 40AH).

I've done an experiment with this before and even if two batteries are left connected for a week they only balance out slightly because the difference between them isn't great enough for a large charge to take place. A normal car battery charges at 14.2 volts. A fully charged battery sits at 12.85 volts - connecting it to a half charged battery which is 12.45 volts doesn't result in much current flow between them.