Safely connecting 2x5S Li-Ions to a 10S battery protector - Page 1
Forum section: electronics / projects

author: robzy
author_rank: Regular Contributor
timestamp: August 08, 2022, 12:55:12 pm
content: So I have 2x5S Li-Ion batteries (5Ah each) that I'm planning to use in series with a 10S battery protector.This is the battery protector that I'll be using:https://www.aliexpress.com/item/32856483718.htmlEach battery has an XT60 and a balance-charging plug (i.e. a wire connected to each cell).My concern is about safely connecting these two batteries in series before the connector. It ought to be fine, but what if I mix up the XT60 and balance-charging plugs. I'll be shorting the two batteries out.Can I protect against this by making a special wiring loom that connects the two 5S balancing charging plugs into a 10S balance charging plug? Maybe with fuses or PTCs on each wire?If so, what's some desirable specs for these high-energy fuses?

author: thm_w
author_rank: Super Contributor
timestamp: August 09, 2022, 09:22:17 pm
content: So you want to remove the two 5S packs and charge them individually?Best would be to keep the entire BMS together, and glue the plugs in place, and charge it at 40V if you are able to do that.If you don't want to do that, then use a different connector type for each pack, or have two dedicated 20V charging connectors and only one XT60 for the 40V output.

author: robzy
author_rank: Regular Contributor
timestamp: August 10, 2022, 01:43:04 am
content: Thanks for the input.To clarify, I actually want to charge them together. I want to treat them as a single 10S battery.The problem I have with that is the danger of making sure that both the XT60 connections AND the balance-charging connectors are connected consistently, so I don't short out the batteries.Attached is the schematic I came up with for the balance-charging connectors to hopefuly prevent a big problem. The datasheet for the polyfuses ishere(60V rated, 140mA, 8ms to trip).

author: thm_w
author_rank: Super Contributor
timestamp: August 10, 2022, 10:30:10 pm
content: Still not 100% sure what your concern is.The XT60 will be soldered from bottom of one pack to top of the other. It is polarized, so should never be connected backwards. The BMS (if you solder the wire to the correct spot) will somewhat prevent from overcurrent scenarios on that connector.If you are worried about this happening while assembling the pack, assemble it without plugging in the balance connector. Measure voltage before soldering. Measure again on the connector once the pack is completed before plugging anything in.Those polyfuse are going to be hell to solder on to wires and get them to stay in place. If you are going to do it, use through-hole poly fuses.

author: robzy
author_rank: Regular Contributor
timestamp: August 11, 2022, 01:58:55 am
content: Quote from: thm_w on August 10, 2022, 10:30:10 pmI have two batteries. Each have an XT60 connector (lets call then OX1 and OX2) and a balanced charge connector (lets call them OBC1 and OBC2).The two batteries will plug into a BMS system that will have two XT60 connectors and two balanced charge connectors (IX1, IX2, IBC1 and IBC2).Everything will work perfectly if I make the following connections:OX1<->IX1OX2<->IX2OBC1 <->IBC1OBC2 <->IBC2But what will happen if I accidentally connect the plugs like this (differences in red):OX1<->IX1OX2<->IX2OBC1 <->IBC2OBC2 <->IBC1I predict bad stuff! Because you will ultimately be shorting the two 5S batteries together. Which maybe the polyfuses will protect me against?Quote from: thm_w on August 10, 2022, 10:30:10 pmI've designed a PCB. Attached photo.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 11, 2022, 02:10:54 am
content: Heat shrink the connectors so they won't come apart easily, then treat the whole set as a 10S pack from now on. (Also verify that the two packs are indeed well matched.)

author: thm_w
author_rank: Super Contributor
timestamp: August 11, 2022, 09:37:19 pm
content: Yeah, as I said, don't have two XT60 connectors, just use one.

author: robzy
author_rank: Regular Contributor
timestamp: August 12, 2022, 04:50:41 am
content: It's all well and good until I (or someone else) needs to service something, needs to pull out the batteries, and makes a bone-headed moveI don't want to permanently bond the two 5S batteries into a single 10S battery, because I might want to use them seperately in the future, so hopefully the polyfuses will help protect the batteries from accidents.