Lithium Ion BMS questions

[moocowman]

This is a battery for my motorcycle, but I'm sure I'll get better answers from RC hobbyists than motorcyclists. I've never had a Lithium ion motorcycle battery last more than a year. It's likely my own fault, leaving the key on accidentally and discharging too much. And hooking it up to plain old chargers. But I've mistreated my lead acid batteries the same and they last 3 years or more. There's no Lithium ion motorcycle battery you can treat like a lead acid battery. I'm wondering if this is because they've left something out? I've opened up a couple of these Lithium ion batteries. I see a BMS in there. I know the BMS balances the cells by discharging cells that have higher voltages until they all match. But do most BMS have the ability to cut off the batteries to prevent over-discharging?

My second question is regarding charging. The most recommended Lithium ion battery for motorcycles is Shorai. They require a special charger, a balance charger I'm sure as it has multiple wires, not just positive and negative. Is there a BMS that will also balance the charging so you can simply hook up a standard charger to the + and - posts and know it's being balance charged? And isn't this important anyway because the alternator charges the battery this way and it's not made to balance charge?

Attached is a pic of the BMS in one of the batteries. Do you agree the only function of this BMS is to discharge balance the cells and it has not over-discharge protection and no ability to balance charging?

[Alpha]

moocowman Welcome!

Typically a vehicle's BMS does more than equalize cell voltage during the recharge cycle, but also during discharge. Smarter BMS units have more features, some have less. Our electric RC airplanes don't use a classical BMS - - rather, we use a simple ESC (Electronic Speed Control) that pulls power out of a battery to moderate a motor's speed according to our throttle control inputs... That's it. It does not balance the battery's cells, and has no role in recharging, since the battery is removed from the airplane for charging and plugged into an external charger.

Any external charging unit that is only "two wires" as you say (positive and negative) is not a balance charger in and of itself but rather relies on the internal balancing circuit within your BMS, which in this hobby we generally don't do. Conversely, an external charging circuit with separate input power and balancing wires, such as what it seems you're describing with your Shorai example, is the preferred route for many RC enthusiasts, especially those using larger batteries comprised of high cell counts.

In any application, RC or not, a lithium battery's cell voltage consistency and internal resistance are two determinants for initial pack quality. How those variables change over time also indicates quality and proper (or improper) use.

The challenge with a sealed multi cell battery in your motorcycle is I'm unsure how easily you can measure internal resistance. A lot of our hobby chargers have a built-in function to measure internal resistance in milliohms. This is one of the easiest ways to determine if a pack is dying and to assess how quickly its effectiveness is degrading. Poorly managed packs will degrade faster than well managed packs. The initial quality of the pack forms the starting point, but all things fade over time. Once internal resistance reaches a certain point, a pack will become practically unusable.

Having toured several battery factories here in China, I can say that initial pack quality is determined by various factors including material quality, cell production, balancing, matching, conditioning, aging, and storage. All of this occurs before an end user receives it, whereupon the pack is subjected to the rigors of every day life in your motorcycle or our model airplanes, etc. Those conditions vary, so my point in all this is that there is no absolute universal rule. Consider the wide diversity of user experience with cell phone battery runtime and lifespan. It largely depends how we use them, and even the best of them won't run the same in 2020 as they did in 2018.

Lead acid batteries have lower energy density, but as you said are less finicky. Lithium has higher potential but does require more careful attention. For starters, I would investigate the quality of your internal balance charging circuit, and replace your brick charger with a dedicated external balance charger if you decide the internal circuit isn't up to the task.

Final note : If your current battery is already toast and its internal resistance already too high, it may be past the point of resuscitation. No amount of balance charge cycles will reduce the internal resistance once a pack is too far gone. Often it's cheaper to start again with a new battery and a new vigilance to recharge and maintain it properly from day one.

I hope this helps!

[moocowman]

Thanks for your response. I though for sure hobby batteries packed more technology, with the amount of money you spend on batteries and how critical it is to keep things from falling out of the sky.

I assumed my dead batteries were toast. I'm not even thinking of trying to resurrect them, because they'll just leave me stranded again. Obviously, being so light and small is a huge advantage. But their characteristics and the environment of a motorcycle makes them unpredictable. Without testing the battery, you can't tell when it's on its way out. A weak Lithium Ion battery will crank the starter just fine because it seems the battery gives it all or nothing at all, unlike a lead acid where you can notice the starter is slow to crank. You get on the road and the battery decides to retire. When that happens, the alternator fuse blows and the engine dies. I never understood why this happens. You'd think it takes less than 30A to run a motorcycle. I imagine it's because on top of running the bike's electrics, it's also desperately trying to charge the battery.

Anyway, I'd like to make my own battery I'm just wondering if such a BMS exists that balances charging and protects from over-discharge on top of its usual discharge balancing function.