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jaco_n
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I received 4 x 100AH batteries that are 3 years old. I would like to install a system that can give me power during loadshedding, about 2 hours 30 min. How do I go about desining it and which is better 12 / 24 / 48 volt? I avarage at night about 600 watt constant usage, sometime 1.5Kw.

Would it be something like this if I want to cater for 1000w for 3 hours to make it easier:

3 x 1000w = 300kw.

3000w / 12v = 250amp

3000w / 24v = 125amp

3000w / 48v = 62.5amp

 

If I connect the 4 batteries as 12 volt then I have total 400amp, work on 50% DOD which is 200amp or 2400 watt

If I connect the 4 batteries as 24 volt then I have total 400amp, work on 50% DOD which is 200amp or 4800 watt

If I connect the 4 batteries as 48 volt then I have total 400amp, work on 50% DOD which is 200amp or 9600 watt

 

Or did I get my maths wrong?

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Hi Jaco

 

If you connect your 4 batteries in parallel and you would have 12V and 400Ah.  (4.8 kWh).

 

If you connect your 4 batteries as a 24V system you will have 2 strings consisting  of 2 batteries each connected in series and the two strings connected in parallel. Now you will have 24V and 200Ah (4.8 kWh).

 

If you connect your 4 batteries as a 48V system you will now have 4 batteries connected in series. Now you have 48V and 100Ah (and still 4.8 kWh).

 

Since you want to work on a 50% DOD you are only going to use half this capacity. So you now have 2.4 kWh.

 

If you use 600w your batteries should last just under 4 hrs.  (Batteries are efficient devices especially below 70% charge but there will be some losses in the inverter etc. 

 

If you draw 1000w your batteries are going to last just over 2  and a bit hours.

 

I think a 48V system is better - you will use thinner cabling (because of the higher voltage) and your losses will be less. You will not need busbars to connect batteries or battery strings in parallel. The disadvantages are that you can expect quite a large spark when you connect and an accidental shock from 48V DC may not be pleasant. My Dad managed to shock himself whilst welding with about 30 VDC. He told me it was not an experience he would care to repeat.  Make sure you connect with a battery disconnect and not near inverter components.

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Hi Jaco

 

If you connect your 4 batteries in parallel and you would have 12V and 400Ah.  (4.8 kWh).

 

If you connect your 4 batteries as a 24V system you will have 2 strings consisting  of 2 batteries each connected in series and the two strings connected in parallel. Now you will have 24V and 200Ah (4.8 kWh).

 

If you connect your 4 batteries as a 48V system you will now have 4 batteries connected in series. Now you have 48V and 100Ah (and still 4.8 kWh).

 

Since you want to work on a 50% DOD you are only going to use half this capacity. So you now have 2.4 kWh.

 

If you use 600w your batteries should last just under 4 hrs.  (Batteries are efficient devices especially below 70% charge but there will be some losses in the inverter etc. 

 

If you draw 1000w your batteries are going to last just over 2  and a bit hours.

 

I think a 48V system is better - you will use thinner cabling (because of the higher voltage) and your losses will be less. You will not need busbars to connect batteries or battery strings in parallel. The disadvantages are that you can expect quite a large spark when you connect and an accidental shock from 48V DC may not be pleasant. My Dad managed to shock himself whilst welding with about 30 VDC. He told me it was not an experience he would care to repeat.  Make sure you connect with a battery disconnect and not near inverter components.

 

Ditto

 

Parallel strings and parallel batteries is risky, especially on "used" batteries, as in your case. If 1 cell in one battery dies (shorts), that battery's voltage will drop by approximately 2V, but the other batteries, connected in parallel with the faulty battery will still be at 12V and will try to keep the faulty battery's potential at 12V and effectively the working cells in the faulty battery will be overcharged by the other batteries at extremely high currents. All the batteries will get warm and you risk the potential of fire and/or an explosion. If you run parallel strings or single batteries in parallel you'll have to add at least a fuse in series with each string or battery as well.

 

The more power you require from your inverter the higher the input voltage should be to have the least amount of losses. Usually the smaller inverters (< 1500W) will work on 12V, the 2-3KW inverters usually require 24V and the 3KW and higher inverters usually require 48V or 96V or even higher. If you are going to draw up to 1.5KW, you should get an inverter with a capacity slightly more than that, say 2KW. Now if you have decided on a specific brand you can check to see if they have different models, e.g. 24V models and 48V models. As per the explanation above, I'll go for the 48V model, but there is one more thing to keep in mind: the inverters' minimum charging current. The best charging rate for batteries is at approximately C10 - that is 1/10th of the ampere hour rate of the batteries at 20 hours - your batteries have a 100Ah (20H) capacity and therefor your ideal charge rate would be 100/10 =>10 amps. If your choice of inverter (48V) now only have a minimum charging current setting of say 20A, then you will be charging your batteries at a too high current. If they have another model inverter (24V) also with minimum charging current of 20A, then you'll have to connect your batteries two-two in series and the 2 strings in parallel (to get 24V) and at 20A charging current each string will then be charged at 10A, which would be better for the batteries.

 

PS. you won't shock if you touch 48V DC at the battery terminals or inverter input, if it was 48V AC (or pulsed DC) you might feel a slight tinkle.  :)

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