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Advice on battery configuration

JanC

New Member
Joined
Nov 20, 2022
Messages
39
Location
Portugal
Hi, I'm just planning my small off-grid system, and I can't decide about my battery configuration. I want to also check, if I took everything into account because I'm a beginner in this.

So I'm going with lithium for sure, and I need about 2400Wh of capacity. At first I wanted to buy 2x 12v 100ah batteries, but I found some good deals on bigger batteries, and I see only disadvantages with buying 2 batteries instead of just one large battery. At least for my situation. So I want to be sure that I didn't miss something.

I'm building something like a tiny home, so it's not gonna be mobile and therefore weight is not an issue for me. It will stay in one place forever hopefully. So with two batteries, I can only see more cables, more losses and more things to go wrong. Also series vs parallel is another decision I wouldn't be sure about. I heard Will say, that batteries like to be in series, and if they are in parallel they wear out quicker, or something like that. And it's not easy to find batteries, that are cheap enough, and can be connected in series. Some people say parallel is better, so I don't know. Having only one large battery would solve this problem for good.

The only problem I found with buying one battery is the maximum discharge rate. If I understand it correctly, when there are 2 batteries connected, their discharge rate doubles. The 12v 200ah battery I have in mind has a maximum continuous discharge at 150a, so that's about 1800w right? It has also a surge discharge somewhere over 400a, so that's more than enough, but I'm planning to buy a 2000w inverter to power a fridge, and I don't know if that continuous discharge rate is enough. It's not that big of a difference, so if the battery has protection, it's just limiting the inverters potential right?

Another good deal I found was a 24v 100ah battery. For my situation, I don't really see any huge differences between 12v and 24v system. The cables to the inverter, fuse box and the fridge will be very short so the wiring cost difference will be minimal. As I think about it now I'm not 100 % sure about this, because there will be some DC lights and extensions I would like to stretch from the fuse box to about 10 metres from the battery. I don't know how much would that change the wiring costs. I have chargers for phones and a laptop that are 12/24DC, so I could use them either way. I have one kettle though that's only 12v. I would probably need a convertor for that. Btw what happens when I connect to a 24v battery something that's only for 12v? Does it only fry the 12v appliance and it's cables, or it can damage the battery, DC outlet, or the fuse box?

So to wrap it up it's either: 1. 2x 12v 100ah
a) in parallel
b) in series
2. 1x 12v 200ah
3. 1x 24v 100ah


I'd be grateful for any advice

Jan
 
Hi JanC - Your inverter will specify voltage and capacity size needs. There are many advantages to higher voltages in PV arrays and batteries. I am using 48V 5.1 kWH 100 Ah batteries. Two of these would likely be sufficient for any instant on demand you would have. You would then calculate your PV size to recharge your batteries while supplying your daily max usage (if you want to be fully offgrid), further gray days would be factored in to supply enough battery backup to sustain during repeated overcast days.

Your intended use and expectations is more important than component details. Remember that higher voltage (pressure) requires smaller (diameter) copper wire size saving some money.

Hope this helps a bit.
 
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