High Cell-Count LiPo Charging

[nemondemon]

Does anyone have a solution for charging LiPo batteries with a high cell count (15S+)? Most balance chargers seem to top out at ~10S and while BMS is an option, I haven't seen any products which have a storage voltage option (essential to extending the lifetime of LiPo batteries).

I have seen the ISDT X16 will charge up to 16S but this comes at great financial expense...

 

[harrisonpatm]

The short answer is, once you get to voltages that high, balance (hobby) chargers aren't typically used. What we've seen done before is people splitting their larger series packs into 4-6s sections, pulling them off to balance charge individually, and reconnecting in series for use. So you could do it that way, if you're dead set on using hobby chargers.

But again, once you get to voltages that high, you're probably better off going with an adjustable-voltage charger, and a BMS that may or may not feature active balancing (if a pack it well build and properly taken care of, it shouldn't go out of balance much, if at all).

As for storage voltage, typically one would just either pay attention to charging and stop before it gets to full, or take a fully-charged pack and use it until the voltage comes down. You can always use inexpensive load testers to bring voltage down to a set point, and those are handy to have around for other uses anyway, like capacity measurement. testing. You can also just rig up an appropriately-sized resistor bank, wired to a connector that matches your charge port, thought with that method you'd have to pay close attention and manually disconnect when it reaches the voltage you want.

Sorry if that's not much of an answer. Though as you mention, you can get some high-voltage hobby chargers, as long as you don't mind the expense.

 

[nemondemon]

The short answer is, once you get to voltages that high, balance (hobby) chargers aren't typically used. What we've seen done before is people splitting their larger series packs into 4-6s sections, pulling them off to balance charge individually, and reconnecting in series for use. So you could do it that way, if you're dead set on using hobby chargers.

But again, once you get to voltages that high, you're probably better off going with an adjustable-voltage charger, and a BMS that may or may not feature active balancing (if a pack it well build and properly taken care of, it shouldn't go out of balance much, if at all).

As for storage voltage, typically one would just either pay attention to charging and stop before it gets to full, or take a fully-charged pack and use it until the voltage comes down. You can always use inexpensive load testers to bring voltage down to a set point, and those are handy to have around for other uses anyway, like capacity measurement. testing. You can also just rig up an appropriately-sized resistor bank, wired to a connector that matches your charge port, thought with that method you'd have to pay close attention and manually disconnect when it reaches the voltage you want.

Sorry if that's not much of an answer. Though as you mention, you can get some high-voltage hobby chargers, as long as you don't mind the expense.

Thanks for your reply; all your points make a lot of sense. When you speak of an 'adjustable voltage charger' please could you clarify the kind of thing you mean by this? A link or two for existing products would be greatly appreciated.

I had wondered what kind of power source people use with a BMS up until now. I thought people merely connected a power supply set to the voltage of the fully charged pack. I assume these 'adjustable voltage chargers' are more complex than this? I always thought hobby balance chargers had a set programme of adjusting the voltage and/or current throughout the charge cycle (making them more complex than a simple power supply).

Please excuse my lack of electrical knowledge; it's not my strong suit...

 

[scianiac]

You probably want to just get a programmable BMS as most of those let you set the termination voltage lower typically via bluetooth and with that you can also set the balance voltage lower. If you just supply a normal "dumb" BMS with a lower voltage it will of course charge to a lower voltage but if that voltage is too low you will never enter the voltage range where the BMS balances and eventually it will be out of balance. Now any cells that get too high will hit that balance voltage but will only be discharged back down to it so imbalance will still exist. And with the programmable BMS you don't need a special charger.

The other solution is to just split the battery into banks small enough to charge with a 6-8S hobby charger.

 

[harrisonpatm]

When you speak of an 'adjustable voltage charger' please could you clarify the kind of thing you mean by this? A link or two for existing products would be greatly appreciated.

What's your desired battery pack? I'll assume 16s. 16 x 4.2v/cell = 67.2v. So head over to your preferred online vendor, and search 67.2v charger. Some of them will adjust voltage, some won't.

Typically this style of charger can be opened up to find a small blue trim pot on the inside. Using a multimeter, you should be able to turn it to adjust your exact charging voltage that you want.

I had wondered what kind of power source people use with a BMS up until now. I thought people merely connected a power supply set to the voltage of the fully charged pack. I assume these 'adjustable voltage chargers' are more complex than this?

Nope. They can be, but they don't have to be. A current-limiting, adjustable voltage power supply can perform the same function as a charger.

 

[nemondemon]

What's your desired battery pack? I'll assume 16s. 16 x 4.2v/cell = 67.2v. So head over to your preferred online vendor, and search 67.2v charger. Some of them will adjust voltage, some won't.

Typically this style of charger can be opened up to find a small blue trim pot on the inside. Using a multimeter, you should be able to turn it to adjust your exact charging voltage that you want.

Nope. They can be, but they don't have to be. A current-limiting, adjustable voltage power supply can perform the same function as a charger.

Thanks again for your reply, this makes more sense now. My only worry now becomes the balancing ability of the BMS itself. I understand that conventional LiPo balance chargers wean current from the cells which reach 4.2V until all cells reach this value? Without this process, I'd assume there's a chance that some cells could be overcharged to compensate for others which are undercharged. Can these BMS units perform this same operation? I understand many are advertised as balancing but unsure exactly how they are programmed to operate.

 

[harrisonpatm]

Thanks again for your reply, this makes more sense now. My only worry now becomes the balancing ability of the BMS itself. I understand that conventional LiPo balance chargers wean current from the cells which reach 4.2V until all cells reach this value? Without this process, I'd assume there's a chance that some cells could be overcharged to compensate for others which are undercharged. Can these BMS units perform this same operation? I understand many are advertised as balancing but unsure exactly how they are programmed to operate.

I mentioned in my first response, that if you have a well-balanced pack, that's healthy and taken care of, you won't need to balance it. If you have a pack that's completely out of whack and needs balance charging on every cycle, then you have problems with the cells themselves and are risking fire.

A BMS should cutoff charging completely as soon as any cell group reaches your set overvoltage limits.

 

[scianiac]

Basically any BMS will balance the cells, most of them by discharging the higher voltage cells and it will start this process at either a fixed voltage that is listed in the specifications or a variable one you can set, but it's often a little below 4.2V and it will do this until the cells are close to within a certain range, again fixed or adjustable. And yes a pack consisting of new and good cells this won't happen much but needs to happen at some point over many cycles. This is why I recommended getting a BMS you can program, because one you can't will only start balancing at say 4.1v for example so if your simple charger is set to say 60.8v for a 16S pack you can see the BMS won't do any balancing until a cell gets to 4.1V while the others are around 3.8v which is way too much of a range. But if you can set the BMS to start balancing at 3.8V and to stop charging at 4V or whatever you want then all is good and you can use whatever charger you like (within reason). Also at this point I don't know why you wouldn't buy a programmable BMS, they are only slightly more expensive and offer a lot more features and ability to monitor the battery.

 

[nemondemon]

Basically any BMS will balance the cells, most of them by discharging the higher voltage cells and it will start this process at either a fixed voltage that is listed in the specifications or a variable one you can set, but it's often a little below 4.2V and it will do this until the cells are close to within a certain range, again fixed or adjustable. And yes a pack consisting of new and good cells this won't happen much but needs to happen at some point over many cycles. This is why I recommended getting a BMS you can program, because one you can't will only start balancing at say 4.1v for example so if your simple charger is set to say 60.8v for a 16S pack you can see the BMS won't do any balancing until a cell gets to 4.1V while the others are around 3.8v which is way too much of a range. But if you can set the BMS to start balancing at 3.8V and to stop charging at 4V or whatever you want then all is good and you can use whatever charger you like (within reason). Also at this point I don't know why you wouldn't buy a programmable BMS, they are only slightly more expensive and offer a lot more features and ability to monitor the battery.

Thanks. Can you recommend a BMS which has these features at a reasonable price point & form factor?

 

[scianiac]

I think there was a thread a few days ago about people recommending BMS brands, I've used the LTT ones and liked them as have many others, they are very reasonably priced, well built, form factors maybe are slightly larger and don't have pretty cases, at some point you want to make sure the BMS is large enough to contain enough FETs to do what it needs to do. The documentation on their website could use some work and figuring out which one you want can be annoying but their customer support is great. I haven't tried to set mine to 3.8V but it seems I can set whatever voltage I want to type in, you could always ask them to make sure.

JKBMS is another brand I've heard good things from.

 

[lnanek]

Some previous threads concluded that it is ok to connect several RC chargers at once to different segments of the battery as long as they all have electrically isolated power supplies. E.g connect one 8s charger to the first half, then one 8s charger to the second half.

There's even a few hobby chargers that support networking/cascading multiple chargers and running them in a sync charge mode. Hyperion NET3 chargers, for example. The manual for those says you have to disconnect the two halves of the pack before charging, though. So it's actually less convenient.