Quote:
Originally Posted by Ross&Hailey
Hi, so for the last several months of our buildout, I had been planning on getting 8 Duracell 6v GC2 batteries. I kept checking Sam's Clubs website and availability was concerning me. DIY LiFePO4 batteries are also comparatively reasonably priced. Sooo I bought 8 304 AH EVE cells (approx 7300 WH). The sales rep told me that they were already shipping to a warehouse in Texas so they told me to expect them within the next few weeks. We will see if that is accurate. Regardless, I need to buy the rest of the components to put together these batteries.
I want to test these cells as soon as I get them in a controlled setting (not in my bus). What is the best way to test their capacity and things? I want to be able to do so within 15 days of receiving them in case something isn't quite to spec so that I can get a refund.
I will be using a 24v system. I felt that doing an 8S configuration would be the best. What do you experienced folk say? Is 8S best? Or would a 4S2P system be better?
The two BMS brands I've been seeing are Daly and JBD. I know I can order Daly from a distributor here in the US. I imagine an 8S 200A BMS would be the way to go? Where do you buy JBD BMS if that is better?
The cells will come with 8 bus bars to connect the cells together. Should I use those or are there better options for a skoolie?
I've seen Will Prowse use a yellow clear tape to bind all the cells together and compress them somewhat. Is that the way to go for putting the packs together?
If you're curious about the rest of my system. I will have 1360 watts of solar on the roof. An LV2424 Hybrid Inverter charger MPPT charge controller combo. None of this is installed, but I have the solar panels and the inverter already.
I would appreciate any feedback from you experienced battery builders out there. Hopeful that all 8 cells will arrive in good condition. I spoke to one other skoolie that ordered 16 of the same cells from the same distributor and had great luck so I am hopeful.
Thanks!
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So when you receive your cells, they will be at a partial state of charge (SOC), probably about ~30% SOC +/-. The will also not be balanced. So before you will be able to do a true capacity test you'll want to balance the cells.
Top Balancing (basic description):
This involves connecting all the cells in
parallel, connect to a proper power supply and charge them until ~3.6 Volts AND current drops away to near zero, at this point your cells will be balanced. Because there will be no BMS protection in this process, its important that you understand what you are doing before you do it (read up on it), and you are present and attentive during the last portion. Here are two links (general
explanation and
guidelines, read both).
This is what top, bottom and unbalanced cells look like in visual terms:
Capacity testing:
At this point your cells are at 100% state of charge and top balanced. Once your cells are top balanced, you can rearrange them into a series configuration and wire up the BMS. You'll need a way to accurately track amp-hours consumed (not a voltage based battery monitor). Some BMS's can do this, as can many battery monitors like the Victron BMV, or a shunt or hall effect sensor. Probably you will have purchased something like this already for your system (since you'll want to know state of charge). Alternatively, you can use a dedicated capacity test like shown
in this video, but those small testers will take a long time for a full pack. You'll want to make sure you set your BMS to disconnect the battery when the first cell hits the low voltage cutoff. That is 2.5V for most lifepo4 cells, you could use a little higher value of 2.6-2.8 if you want, it'll skew capacity a little bit, but not much since you are in the lower knee.
Alternatively you could use one of thos little capacity testers in the video to test each cell individually, in which case you could do that first, before top balancing. As you can see there is more than one way to do this (and a lot of wrong ways
) to some extent it will come down to what components you have on hand/are convenient to you.
As to taping your batteries, I wouldn't personally do that, there are better ways to secure batteries, Will's old tape method was one of the most commonly criticized things in this videos. Remember he is demo'ing usually free cells on a bench, and trying to keep things as simple as possible. As to how to properly secure your cells, you can get as nerdy/professional or as basic as suits your personality. If you ask your manufacturer (EVE) they will state that cells should be compressed at a constant 12PSI / 300KGF for maximum longevity. They have fancy ways of doing this. A DIY way is to use two pieces of plywood, threaded rods, and springs calculated to apply the proper force. Its a little technical, lots of people opt not to go that far but its growing in popularity. In the spec sheet you should have for your cells, they will list one cycle life specification for compression, and one for no compression. Both are over 2500 cycles if I recall correctly, but compression supposedly adds at least 1000 cycles. DIYsolarforum, is where you would want to go for more info on securing/compressing cells.
As to BMS, between the two, I would choose the JBD BMS, you can buy from a US based seller called "overkill solar," they have a website, and also sell on amazon. This is my top reccomendation for a BMS for beginners/less technically inclined folks. It has a bluetooth app, its fairly simple, it has good documentation (written by overkill solar), and the seller I mentioned offers good beginner friendly support if you need it. The only reason you might choose Daly over JBD/Overkill, is that the 8S JBD BMS is limited to 100A, if that is not enough (100A @ 25.6V = 2560W) you will need to look elsewhere. Daly is not a bad BMS either, and is available in higher current ratings, you definitely want to oversize it though (add 20% to 50% to the size you think you need). Both daly and JBD are cheap chinese commodity BMS's, but they are two of the brands with decent track records. JBD wins out when it comes to documentation/finding community support/help.
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