Advice on building 60V 120-150 Ah LiFePO4 battery pack with charger - Page 1
Forum section: electronics / projects

author: globoy
author_rank: Frequent Contributor
timestamp: March 10, 2024, 06:25:57 pm
content: Read at the bottom if you want the non-technical details but as the family "techie" I have been asked to source a replacement battery pack and charger for a super-cheap imported Chinese electric food truck. The vehicle has a "4 kW" motor and came with 5 12V 120 Ah lead acid batteries (60VDC system) and a super cheap charger. I was asked to get involved after damage was done so now the battery pack has to be replaced and I think the charger should be replaced too.I have found high quality lead acid battery chargers and know I can source some decent 12V 120 Ah lead acid batteries but I want to explore using LiFePO4 batteries instead for their longer life (and it appears their price has come down significantly making them competitive with lead acid - even more so because of their longer life). And I'd like to explore a larger capacity pack build using 150 Ah capacity or higher batteries. The charger will be North American 120V 60Hz.There are lots of 12V LiFePO4 batteries at all kinds of capacities. They all appear to have some kind of built-in BMS. However I am unclear if one needs a special charger for them, and especially how one would go about charging 5 batteries wired in series. Would their built-in BMS be able to handle being connected to a 60V lead acid charger (bulk/absorption + float)? Or would I need a special charger, perhaps one that manages cell/battery balancing? One reason I ask is that I found several 4-packs of batteries on Amazon and they all mention you can charge "up to" 4 batteries in series. An example is shown belowhttps://www.amazon.com/CHINS-LiFePO4-Battery-Lithium-Built/dp/B08N533WNL/ref=sr_1_3Any suggestions for batteries and chargers would be appreciated. I'd like to see if I can use existing battery packs (e.g. "12V" batteries) instead of custom building a battery pack out of individual cells. I think that a 100A continuous discharge rate will be ok (probably has to support higher transient discharge rates) and something like a 20A charge rate (e.g. overnight).Backstory (TL;DR part)I was asked by my son-in-law to talk to his sister who was having troubles with her food truck. For the purposes of this story I'll call her my DIL. She has been converting this super cheap Chinese vehicle she imported to both get a license to drive it in the USA and to get it to pass the food safety inspections. I sort-of had followed this from a distance but had no idea where she was. It's kinda crazy. She bought the thing for $5kUSD...but then spend about $15k getting it shipped, imported and delivered (of course all that was required was never mentioned when she bought it and she didn't understand how shipping/importing really works). Despite all that she has managed to get most of the way through being able to legally drive the thing and prepare food in it.It's a bit of a crazy vehicle. It kinda looks like a trailer on tiny wheels. It has a tiny "4kW" motor (that's the marketing, I don't really know what the motor is) and a claimed range of 35 miles at up to 35 MPH. I don't think it ever went either that far or that fast when my SIL drove it around initially. However I guess the low range isn't an issue because my SIL only plans to drive it a few miles in her town to various places food trucks congregate. And the low speed isn't [too much] of an issue because it's all in-town roads.I got asked to take a look at the electrical system but had no idea what that meant until I got to her house. They wanted to know why it would only go a few miles now. It turns out that they been trying to "fix" the battery system themselves. My DIL bought a pair of bargain basement cheap DMMs (think not as good as Harbor Freight models) and then proceeded to make all her measurements on the VAC range, which on these meters displays about double the actual DC voltage. This lead her to decide to remove one battery which showed a lower voltage and run the system on 4 batteries instead.The 220V charger is essentially a big transformer and a diode. The transformer secondary is tapped at a few points routed through a rotary switch to set the float voltage. There's also another tap and small diode which powers a 12VDC fan and some LEDs (whose function I didn't ascertain). The rectified output goes through a shunt with a meter across it functioning as an Ammeter. Range is 0-30A. The Chinese company had "helpfully" shipped a small 110->220V transformer (500VA rating). Of course this is woefully undersized and burst into actual flames at some point. I think they then tried to connect the vehicle to 220V but it all seemed a bit hazy in the recollection. The removed battery actually reads about 7 volts and the remaining in-circuit batteries about 12 volts. I suggested taking the remaining cells into an auto parts store to get them tested but my DIL just wants to replace them and the charger - which is probably the right thing to do.As one might imagine I was wildly bemused as I actually came face-to-face with this crazy vehicle. It originally came with a boxy stool as the driver's seat. My son-in-law welded in a car seat in order for it to pass a vehicle inspection. There's a little LCD module in the dashboard that displays battery voltage/level and speed. It has a 60VDC to 12VDC buck converter module to power all the various lights, horn, etc. It has sinks with demand hot water, a small fridge and freezer and a dual gas oven. There's an onboard AC electrical distribution system with plugs that support both the North American two prong 120V and a three prong 220V plugs. It's actually kinda nice inside as a food truck and the body looks reasonably built (like a good trailer). I'm kinda shocked to hear that it can be made street legal in the US but I guess it will be. Certainly it's good my DIL will only drive it on some slow moving, low traffic suburban roads. I worry about the undersized motor and want to take a further look at the wiring but I do sort-of admire the hutzpah of the Chinese entrepreneurs who built it.I would have recommended against ever going this route but now after a bunch of sunk costs I would like to try to help my DIL get it running.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 12, 2024, 11:28:52 pm
content: If you need 60 V rated lifepo battery then you typically need 5x4 = 20 elements controlled by one single BMS but not 5 differen batteries with 5 BMS. That's how it works