Circuit Breakers on Batteries in Parallel

[ch2]

Hi,

I am working on a Environmental Monitoring Sensor Network that will be going out on the Great Barrier Reef off North Queensland, Australia. For more info on the project, have a look at this page.

We are going to have a small computer, some sensors, a web cam and a microwave link on the reef. This will be powered with an AirX Marine turbine and a 500W array of solar panels charging 6 x Vision 6FM100 AGM batteries. There is a circuit diagram here.

My question is about the circuit breakers (labelled 2 and 3 on the diagram). The main thing I want to protect is the battery bank. In the unlikely event that a battery goes closed circuit I don't want it to allow the others to discharge through it. What rating should I have for the circuit breakers on the leads to the positive battery terminals? The maximum combined current from the turbine and solar panels should be ~60 Amps. I have six batteries in parallel sharing that load. I would have assumed that would mean a  maximum of 10 Amps going into each battery during charging, and that a 25 Amp circuit breaker would do the job. However, I have been told that batteries don't always behave identically and that it is possible for one to take a lot more current then the others in certain circumstances. Is this true? What maximum current should I allow for?

The solar charge controller is a http://www.plasmatronics.com.au PL40.

Thanks,

Cameron  

[Tom in NH]

I don't think you need all those breakers between the turbine/solar panels and the batteries, although the breaker (or a fuse and switch) between the batteries and the load is vital.

If you must have them, I'd suggest an amperage that is double the maximum output of your power generation equipment. You want to make sure it isn't too small and breaks everytime you get a gust of wind on a sunny day. Make sure you choose a wire gage sufficient to handle the current that will go through the breaker. --tom

[Opera House]

If you are concerned about one battery discharging all the others, I would isolate each battery with diodes.   This doesn't solve the problem of a shorted battery sucking all the power that is generated.  A breaker in series with with each diode on the charge side would work if it was sized close to the maximum current ever expected.  This of course assumes that sufficient charging conditions exits before the other batteries drain from normal operation.  You might want to consider using PTTC resetable solid state fuses.  These won't disconnect, but will drop the current to about 1/10th the trip rating.  This would allow sufficient charging of the rest of the batteries.  Suggest battery monitoring along with the rest of your data.

[cyplesma]

If it's going to be truely remote and your not going to visit it for weeks on end.

I would consider putting two relays on each batterys positive terminal. they would need to handle the amps going in and out. put a volt monitor across each battery that can also be switched on/off. or use a single volt meter that can be switched to any single battery at a time.

the two relays at each battery are for:

relay A.) for charging battery

relay B.) for using the battery to supply power.

there's also a manual shutoff switch. diodes, fuses, etc.

it's quite a bit overengineered probably, but I plan on using this as a guide to doing my battery bank. especially since I want to monitor and control what's going on from a panel (scrouged out of dumpster, along with a couple hundred feet of 12v alarm control wire)

where in your case you probably would like to monitor and control all this from some type of pc/whatever computer your using to log all the other stuff your doing there.

[ch2]

When you say double the maxuimum current ever expected, would you use 120A for each battery or 20A (120A over 6 batteries in parallel)? The maximum current for the turbine and solar panels should be about 60A.

Thanks,

Cameron