I knew when I did a drive-by of the site and saw this post I'd get sucked in.
I haven't got much time to explain it all out, but there's pieces mentioned in the other post that should get considered.
The SLA will give you the most range for the $, but the size & weight can become an issue.
LiFePO4 are great, lightweight for the AH, but expensive. Also, they have definite charge & discharge requirements. Discharge them too far, you've got a paper weight. After charging a pack, the charge between the cells needs to be equalized, taking time, wasting power, and the resulting pack doesn't have full capacity. The safe & easy way to build a pack is to have a Battery Management System build into the pack. It protects against over charging and over discharging, and some will charge balance after you've charged. Some will regulate the charge to each cell while charging, so the cells are already balanced when the pack is done charging. E-bike packs used to be readily available in 24VDC, 36 VDC and 48 VDC. AH ranges from 10 ah through 20 ah, and above, but it can quickly get expensive. Assembling a DIY pack can be difficult or easy, depending on the cells you choose.
The cells and the BMS I choose appeared to be the best available at that time. I haven't checked recently to see what has changed.
48 VDC & 10 AH, BMS & case & pack monitor came in around $800 DIY.
In 2010 I built a 48 VDC LiFePO4 using Headway cells.
If I trust the old spreadsheet I dug out, the Max/Min voltage of the pack is 43.8/24 VDC.
I really like that the cell is fully enclosed, and built into one end of each cell, is a space to allow the cell to expand so there is less likely to be a leak due to expansion failure.
No soldering, small screw-head bolts do the connections. Robust connects with no solder joints to fail.
You can use various bus-bars bolted between the cells for series/parallel configurations. They also have plastic brackets that connect to each other for easily building arrays of cells to pack.
No issue getting the cells, as I bought from a North American supplier, who had brought them in from Hong Kong in bulk.
For BMS (battery management system), you can try to do that manually, or use a simple one that monitors the pack voltage or one that monitors each cell in the pack. The per-cell ones are safer, and can be smaller, cheaper, even while having more features.
I used a 5-to-13 cell BMS in my pack that monitors each cell. I specified the max & min cell voltage, max charge current and maximum discharge current, the number of cells, and the board was custom populated to those requirements (oddly the board is configured internally with a positive ground, but interfaces with the world on negative ground...). It's small, compact, and with a harness connected to the board that puts a lead to the junction between each cell: it monitors the voltages and allows for fully charging each cell without over-charging & without requiring a post-charge rebalancing, and stops discharging if a single cells reaches the cell limit, and limits the pack output current. This BMS also has a temperature sensor that gets taped to one of the cells to protect against over-heating while charging or discharging.
You need a specific type of charger that matches the voltage curve to what LiFePO4 works well with.
My knowledge is dated. I would strongly suggest that you NOT
rush out to duplicate what I did in a 36 VDC pack, but it does give you an idea of what to consider when looking at what's currently available/recommended.
I'd suggest starting here: http://www.endless-sphere.com/forums/viewforum.php?f=14
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