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smalltorque asked

Multiplus 24/2000 & 550AH bank

Hi all,

I have a multiplus 24/2000 50/30 & a 150/50 MPPT with 1200W pv array & 340AH 24v bank.

I’d like to replace my batteries with a 550AH 24v bank & keep DOD below 30%.

What I would like to know is whether or not it would still suit my multiplus having the higher capacity bank or should I keep the bank a little smaller, say around 450AH?

Thank you for any information & feedback.

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1 Answer
Mark avatar image
Mark answered ·

I see no real problem with the larger battery bank size in comparison to your Multiplus being only a 2000VA unit.

Its more about how much energy you will use over a full day/night to size the bank, in conjunction with your intended daily DOD (and cost) - rather than the peak power output of your Multiplus.

However, with lead acid based batteries its good practice to have enough charge power (PV or AC charger) to normally charge with a charge current >10% of the battery capacity during the 'bulk' phase. So >55A for a 550Ah battery bank.

So with 50A MAX charge current from either MPPT or Multiplus AC charger, you are probably a touch on the low side.

Particularly if you normally only use the MPPT to recharge, since you don't have that much PV power. 1200W / 28.8V = ~42A MAX. So on an 'average' or 'bad' day (such as during winter) you would get far less out of the PV.

If you were to 'ovesized' your PV array (say 2400W max), then the MPPT could charge with its max charge current of 50 A (or close to it) more often and it would be a better match. But even then probably a bit on the low side. It also depends on what the typical solar conditions are like where you live.

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smalltorque avatar image smalltorque commented ·

Hi Mark,

Thank you for your reply. Yes, sorry I didn’t clarify my question that well, I was actually asking in terms of the charge capacity of the multiplus rather than the inverter capacity.

So if I instead use a 450AH battery bank instead of 550 it will mean the charge current can be at or slightly above 10% when using the charge function of the multiplus but as you stated I would still have to increase my PV array to achieve that 10% with the MPPT. Can you tell me, does it harm the batteries to be charging at less than the 10%? Does it decrease their cycle life or capacity in some way?

Thank you again, Paul.

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Mark avatar image Mark ♦♦ smalltorque commented ·

Hi smalltorque,

I don't proclaim to be an expert on the 'chemistry' side of things, but this is my simplistic understanding - there are 2 reasons to design the system with a minimum charge current of >10% battery capacity for lead acid based batteries;

1- To ensure that the battery is recharged within 'reasonable' time-frame and also to ensure that it is FULLY recharged each day/after every use (within reason)

2- To provide enough current to remove/control sulfation of the battery plates and also to ensure that the charge voltage can penetrate deeply into the center of thick battery plates

So yes I think that this can have some level impact on overall battery life, but there are also many other factors to consider and I wouldn't be overly concerned as long as your charger can provide a charge current of ~10% (or more) and the battery is fully recharged as promptly as possible after each usage cycle (not left in a partially charged state).

Some related extracts for lead acid based/AGM deep cycle batteries (from pdf spec sheets/manuals that I have accumulated);




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