Hi all. I don’t like owning anything “technical” without knowing how it works. My Swytch batteries are maximum 42v fully-charged, 30v discharged. This leads me to believe I’ve got a chain (or multiple chains) of 10 cells in series. When I used to to RC models the battery packs had balancing connectors so that the battery charger could sense and adjust the charging to ensure that all cells were equal. On the Swytch battery their are just two charge connections across the whole pack. My question is, does the BMS inside the battery, monitor and balance each cell, or does it slap the whole voltage across the chain and hope?! Sorry for a such a basic question but I will learn!
BMS question
- Thread starter Cisco-man
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Which model Swytch do you have? I only know that my Mk1 Swytch kit with a 10Ah battery had 4x10 18650 (2500mA)cells and had a BMS which supposedly balanced the 10 groups of 4 batteries (4P10S). The battery failed after about 18 months but I was able to recover about half the cells which were still good. Does that help?
Hi Peter. I use the Max battery packs. I’m not sure what cells are in there, but probably fewer sets in parallel as I have less capacity. Just wondering how good the BMS is at keeping them all balanced.
The BMS monitors the voltage of each cell group (possibly groups of 1 in your Swytch!) and turns on a bleed resistor across each when it reaches a threshold voltage. This slows the charge rate and should allow the other groups to catch up.
But the bleed current is much smaller than the charger output, so the BMS cannot fix large imbalances. If a big imbalance happens, there is a fault somewhere. Well matched cells assembled well will stay pretty close to perfect.
But the bleed current is much smaller than the charger output, so the BMS cannot fix large imbalances. If a big imbalance happens, there is a fault somewhere. Well matched cells assembled well will stay pretty close to perfect.
You have the same balance connector with one wire from each cell-group in your Swytch battery. It's connected to the BMS. The BMS opens a path to ground through a bleed resistors for each group when they reach around 4.15v, so balancing only happens at the top of the charge, which is why you must always charge to 100%.
So I’ve just been for a ride, and both my packs are 36v(ish). I was just going to give them 30 minutes on the charger to get them to around 38v, and store them. From what you say Saneagle, I’ll be wiser to charge them fully before putting them away - even though a 38v storage would be less stressful for the packs. I’m thinking that when I start building my own packs I’ll bring the sense wires out, but might change my mind as I learn more!
The one time not to fully charge is when preparing for long term storage! Leave them at 36V.
I use them once or twice a week, so not planning a long term store just yet. I charge them fully just before use, so I guess they generally should balance up ok if I store them for two or three days at 36 - 38v.
Thanks all for the advice - gratefully received! This is a great forum.
You can charge them up to 38v and leave them like that until you're ready to use the bike, but you should fully charge them before you use it.
I can’t help but feel we’re missing a trick here. RC model battery packs tend to be smaller and simpler but the sense wires from each cell come to the outside world. You know the state of each cell without busting the pack apart. I can see that having (say) 40 cells in a pack you wouldn’t want 30-odd sense wires coming out of the pack, but surely its better to have SOME external monitoring of the cell state rather than none? I’m sure the BMS does a good job of balancing when the pack is fully working but it seems (to my beginners eyes) that it gives no information when the cells start to fail. Surely either bringing out some strategic sense wires to aid non-invasive diagnosis, or even having a BMS with Bluetooth to tell you the state of each cell - without wires would be useful? There’s probably something I’m not seeing yet…
The cells are in parallel groups, and only the groups need balancing because paralleled cells behave like one big cell. So for a 10s 4p pack, there are only 11 balance wires, including the ground, not 40.
There is risk in exposing them because they could be shorted which either destroys the balance wire or causes a fire, so not acceptable for a consumer product without additional protection against harm, additional cost and so on.
There is also no need to be any more sophisticated in charging and balancing than following the manufacturer's instructions and having a properly made battery. It works, so from a mass market customer's point of view, doesn't need fixing.
Hobbyist stuff is a different ball game: knowledgeable users for a start. I don't know much about RC batteries except they are designed for very high current relative to capacity, and for electric flight weight needs to be minimised. Two differences then: can't afford to carry the burden of the BMS during use, and perhaps more significant balancing is needed?
There is risk in exposing them because they could be shorted which either destroys the balance wire or causes a fire, so not acceptable for a consumer product without additional protection against harm, additional cost and so on.
There is also no need to be any more sophisticated in charging and balancing than following the manufacturer's instructions and having a properly made battery. It works, so from a mass market customer's point of view, doesn't need fixing.
Hobbyist stuff is a different ball game: knowledgeable users for a start. I don't know much about RC batteries except they are designed for very high current relative to capacity, and for electric flight weight needs to be minimised. Two differences then: can't afford to carry the burden of the BMS during use, and perhaps more significant balancing is needed?
You don't have sense wires coming out of your phone, laptop or electric drill, nor do you have them in any other houshold rechargeable device. They're more or less limited to r/c models, where you need lighter weight for flying.
Ah, the why has no-one thought of this before syndrome.
Perhaps, a different thought process; eBike batteries have been around for a long time and there are a great many people out there who know a great deal more than me about them and their use, so there really must be a good reason why adding a heap of wires to externally monitor each cell bank is not done.
So use one if you want to.
For ebike batteries one needs one of the BT/uart BMS's if you wish to monitor cell groups closely. Cost is greater as is BMS size , one reason why they are used generically .
I’ll take a look. Not to buy - just understand what technologies are available - thanks.
I don't trust cheap BMSs (as reiterated by others) so I'm rebuilding my recovered 18650 cells from a failed Swytch battery as a twin 4P4S set (ie 2 x 12V). I'm dispensing with the integral BMS and incorporating an RC type monitor (8 inputs) to dynamically check the voltages on each of the parallel sets of cells. Each of those paralled sets will be recharged separately with a simple 1A 3.6/4.2V Liion charger. The resulting battery packs are for general 'cave' use eg desk lighting and occasional friction drive booster use (24V) on one or other ebike. The build is going slowly because of the need for care in handling/charging and the manual balancing of paralleled cells - I will let you know the overall outcome in due course. All a bit of a faff of course but an interesting learning exercise.
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