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Newbie battery questions


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Hi all


We're are newly off the grid (I think it's almost 3 weeks now). I find that I have more questions as time goes by and can't always find clear answers! So if anyone has the patience to explain in technical terms of less than three syllables, here are a few of them ...


The system comprises:


16 x 305W panels (4.8kW)


5kW inverter

24 x 2V forklift batteries (either 640 or 720ah or something like that)

One 4kW petrol generator


I thought somehow it would be easy to read off a meter or a monitor to etermine the state of charge of the batteries, but the value on the MPPT never makes any sense to me. After a good charge, the value will be 58.4V. Then the machine goes into a float cycle and the figure drops to 49V by bedtime, even if we're drawing little power. Why?


I was told to start the generator if the reading fell to 46. This week after many cloudy days it fell to 46.7, so I started the generator (which is wired into the inverter),  but an hour later the reading had dropped to 46.5. When I stopped the generator and checked 30 minutes later, the reading was 57.7. Does it just take a while to reflect the increase ? How long should a generator run to do a meaningful recharge?


If I wanted to not use more than 80% of the battery capacity, how do I determine that from reading the MPPT?


Thanks a mill


Mossel Bay



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What a great system. Must have cost a packet!!!

Could you post some picks please?


For just another R2k you can buy a Victron Battery monitor.

Using just voltage for such an expencive install is not really the way to go.


The initial high voltage is your "bulk" charging cycle.

After a couple of hours of bulk charging, it will change to "float" which will be a lower voltage. This is like a "slow soak" or absorbsion charge.


It is extremely important for your installer to program the MPPT according to the correct battery specifications!!!

If this is not done correctly, you can destroy a very expencive battery bank in no time :(

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Congratulations , this is really a nice system you have installed. I am not an expert but can only provide help from my own experience, so i have a few questions for you.

  1. Was this house on the grid?
  2. What do you use for water heating and cooking?

Like Mike said these batteries needs to be checked, hope they did not have too much shelf life, also your MPPT charge controller needs to be set-up properly. If it goes from one mode to the next too quickly, the batteries wont charge properly.


You will have to get a KW meter to check what your power consumption is on the AC side.  From there you'll get a better understanding on what your big power consumers are?

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  • 5 months later...

How does one determine what size cable to use for linking batteries?  My neighbour insists that my 30mm^2 cable is too thin and that it should be 70mm^2.  This seems way overkill, not to mention R155/m


The thicker, the better. You'll have to give more detail though:

1) Inverter make & model or max input current (from battery)

2) System voltage (if you have not supplied inverter make & model)

2) Distance between battery bank and inverter

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Ok, so you running a 48V battery bank.

Invertor is rated 5kw.

This gives us a max constant battery current of 104Amp.

According to SABS codes, anything from 25mm2 single core welding cable will be fine.

Even at 100Amps you only looking at 0,175V per meter volt drop.

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