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Best way to double battery capacity

Starry-Nights

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Nov 4, 2019
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I think I figured the best way to double the battery capacity of my existing system, but let’s see if there is something I am overlooking.

I built the existing system in summer 2019 using Will’s 24v system video outline with eight 100 AH LiFeP04 Fortune cells (months before Will discovered them!). Have four 100 watt panels on the roof (could squeeze in two more 100 watt panels), going to a 30 amp EpEver controller plus have a 3000 watt pure sine low frequency inverter/charger and a Sterling B to B that can output 35+ amps @ 24v from the 12v alternator. Built my own low-temp shut off and battery heater. System has been bulletproof reliable. (Image attached)

I live in New Hampshire but now winter in Arizona, living full time December to April boondocking off-grid. I’m entirely electric, no propane. There are some winter days with little sun where I need to cut back on electric use. More solar wouldn’t help, thus the interest in more battery.

Think usual solution is to add another set of eight 100 AH cells, but it is my understanding I’d need to add a second charge controller, BMS, thermostat and heater. Existing battery space would also need a difficult modification.

An easier way seems replace the current battery with eight new 200 AH cells. Seems like this would be a drop-in replacement as they could fit in my existing battery space unmodified and I should be able to use the existing heater and low-temp thermostat.

Pricewise, the difference between eight 100 AH vs. eight 200 AH cells is around $400. But, I’d need to add a charge controller, other components and make modifications to my battery compartment to make it fit. The 8 new cells seems a better choice. Plus, I’d have a lightly used set batteries to keep as a back-up or sell.

Comments very welcomed, plus any recommendations on best current deal on quality 200 AH cells.
 

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Think usual solution is to add another set of eight 100 AH cells, but it is my understanding I’d need to add a second charge controller, BMS, thermostat and heater.

No need for a second charge controller unless you add panels.


An easier way seems replace the current battery with eight new 200 AH cells. Seems like this would be a drop-in replacement as they could fit in my existing battery space unmodified and I should be able to use the existing heater and low-temp thermostat.

Pricewise, the difference between eight 100 AH vs. eight 200 AH cells is around $400. But, I’d need to add a charge controller, other components and make modifications to my battery compartment to make it fit. The 8 new cells seems a better choice. Plus, I’d have a lightly used set batteries to keep as a back-up or sell.

Comments very welcomed, plus any recommendations on best current deal on quality 200 AH cells.

Why stop at 200Ah? What's the largest size that fits in your box?
 
Thanks for the replies!

Really thought 2+ year old battery cells should not be paired with new ones on the same controller especially if different brand. I believe current 30amp controller should be able to handle 2 additional 100w panels on roof, bringing total rated to 600W @ 24v (almost never achieved) which is 25 amps.

As for room in my battery space, I've made lots of measurements with different cell dimensions. Eight 200 amp cells would fit, but stepping up to eight 280 amp or larger ones doesn't, nor does adding eight 100 amp cells to my existing eight. Doubling storage seems to be all I should need.
 
winter days with little sun where I need to cut back on electric use. More solar wouldn’t help,
With series panels you can get a bit of charge on low-sun days that you wouldn’t in parallel because the sum volts can be enough for the charge controller to do something with.
Going to 600W in two series strings (3S2P) could help. Here in Vermont I noticed a difference. Not an earth-shattering difference but some watts when I formerly would have expected essentially none. (<0.5A at 12.6V is a number but it doesn’t really do anything useful lol)

Just my thoughts. Add some panels and scale up the battery. Good luck ?
 
The few cloudy days I’ve dealt with in AZ have gotten me about 1/6 th normal production,

If your batteries are fine in capacity for a sunny day, I’d consider just adding more panels, or getting a 50 amp AC to DC converter and run that off a generator for a couple of hours, provided your battery can handle the amperage. 50% the 20 hour amp rating on a lithium shpuld be good, but I don’t have the spec sheet.
 
Good point on voltage. Currently have my panels in 2 serial strings and had intended to add another set of two and thus keep the same output voltage (usually just over 30v). Can see it could be better to increase the voltage with two 3-panel serial strings. I have noticed that I do better with my current series string than others with 400W in parallel.

In winter AZ my lowest SOC is about 65% in morning after cooking breakfast. If sunny, usually achieve 100% SOC by 2PM in January or by noon in late March. I do constantly check my SOC before using any hi-amp draw items. If there is a string of cloudy days, I cut back on energy-use so have some reserve. I'm in a van and don't want or have space to add a generator. In an emergency, I can go for a drive and use the Sterling B to B which my current meter shows delivering 35+ amps at about 28v. (Ideally, plan shopping drives for cloudy days.) Feel more battery should give a buffer and lessen concern on cloudy days but adding both battery and panels better.
 
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