Do I need the 6th Battery?

[TonyK]

My system has 4, 100 watt panels, a 40A MPPT charge controller, a 900 watt inverter, and 6, 35 AH lead acid batteries. One battery has gone bad. The others are only 3 years old and working great. What I want to know is this. Given this system with 4, 100 watt panels, and its charging capabilities, was 6 batteries the right number for this system in the first place, or can I eliminate the bad one and not replace it? I live in St. Louis MO and only use it for just occasional short term emergency power for a tv, computer, chargers for my devices, and a few lights. It has done well for that so far. I’m not sure if I really need or should have 6 batteries. Is it ok to add a new battery with the old ones? Thanks

 

[sunshine_eggo]

400W/14.4V = 28.3A

28.3A is a healthy charge current for 218 (.13C) - 471(.06C) Ah worth of batteries with a sweet spot around 280Ah.

 

[TonyK]

400W/14.4V = 28.3A

28.3A is a healthy charge current for 218 (.13C) - 471(.06C) Ah worth of batteries with a sweet spot around 280Ah.

So based on what you are saying I could actually use up to 7, 35AH batteries if I wanted to?

 

[DIYrich]

You set the charge controller for the rate to charge the batteries. Whether you have enough batteries:
1) enough to supply the peak load?
2) enough to supply needs for x hours or days.

A related question is if you have enough panels to recharge the batteries. At least for daily need, or enough to recharge over two days to replenish extended draw period (bad weather, grid down).

 

[sunshine_eggo]

So based on what you are saying I could actually use up to 7, 35AH batteries if I wanted to?

I would say that's a minimum. Fewer than 7 may mean they are charging at a higher and more stressful current.

 

[TonyK]

I would say that's a minimum. Fewer than 7 may mean they are charging at a higher and more stressful current.

So I did get a 6th battery. That is about all I can fit in a very limited space and keep all the connecting cables the same length, which I'm told needs to be done. Would it be wise to take one of the solar panels off line so I don't stress the 6 batteries?

 

[SamDeleted]

So I did get a 6th battery. That is about all I can fit in a very limited space and keep all the connecting cables the same length, which I'm told needs to be done. Would it be wise to take one of the solar panels off line so I don't stress the 6 batteries?

It depends how your panels are mounted and what they actually produce.

If say your 400w of panels is flat mounted on a van roof , you'd be lucky to ever see 250w out of them at any one time

250w/14.4v = 17.3amps , so by eggo's maths the sweet spot would be 173ah~


You have 5 x 35ah batteries = 175ah


Close enough for me, I think you should be good!

 

[sunshine_eggo]

So I did get a 6th battery. That is about all I can fit in a very limited space and keep all the connecting cables the same length, which I'm told needs to be done. Would it be wise to take one of the solar panels off line so I don't stress the 6 batteries?

In addition to what @SamG340 mentioned, it depends a lot on usage. If the system is lightly loaded, and/or the batteries are typically recharged before noon, then the array will never output peak current, and you should be fine.

Basically, how often will these three things happen at the same time:

1. Batteries can pull full array output for charging
2. Solar conditions actually yield maximum array output.
3. No loads are pulling power from the array.

If rare, you're good.