Attempting to understand Lipo charging - Page 1
Forum section: electronics / projects

author: tyguy2
author_rank: Unknown
timestamp: November 25, 2016, 10:54:27 pm
content: Hello all,I've been working on a project that requires 2 lipo batteries, but I've been confused on how to charge them (and no, I can't just use one battery). At first, I though I could just charge them in parallel, but a lot of forums warned against that because it hurt battery life in the long run, and could be dangerous if the batteries were not balanced. I looked into purchasing a balance ic, but I couldn't quite find anything I understood. I foundhttp://www.issi.com/WW/pdf/31PW8200.pdfbut I don't know if I will need the use the I2C port, and I'm not sure how it will work with VCC disconnected (ie, when it isn't charging) Maybe there's something I'm missing. Could anyone offer some ideas for charging these 2 batteries, or explain to me how the IS31PW8200? Could my device be charged and used at the same time using IS31PW8200?Thanks

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2016, 11:28:18 pm
content: Charging lithium ion batteries in parallel is fine if the manufacturer data sheet says it is OK. Normally two can be paralleled, sometimes three. It is important to be informed and know what you are doing (for example, make sure both batteries have the same voltage before you connect them). If you put two batteries in parallel you get a battery with the same voltage but twice the capacity. I think it might be necessary to de-rate the CC current value when charging parallel cells in case of unbalanced charging current (e.g. use the current suitable for a single cell, don't double the charging current).LiPo cells present special reasons for caution due their tendency to catch fire if something goes wrong. So anyone who doesn't know your skills will advise you not to do anything out of the ordinary just in case. That said, a LiPo cell is mainly a lithium ion cell without a metal case and the chemistry is much the same. Whichever way you can charge a lithium ion cell you can charge a LiPo cell.

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2016, 11:31:55 pm
content: Incidentally, you can tell that putting lithium ion batteries in parallel is fine from the existence of many commercial battery packs (laptops and power tools) that have cells connected in parallel. However, it must be assumed that these battery packs use cells designed for this application. It doesn't mean that just any old random batteries can be paralleled.

author: tyguy2
author_rank: Unknown
timestamp: November 25, 2016, 11:55:55 pm
content: Quote from: IanB on November 25, 2016, 11:31:55 pmSo if I usehttp://ww1.microchip.com/downloads/en/DeviceDoc/22036b.pdfwith 2 batteries in parallel, and not have any issues, as long as the batteries are designed to do so? Should I include protection circuitry such as thishttp://www.diodes.com/_files/datasheets/AP9101C.pdf?What if I attempted to draw power from the batteries as I charged them?

author: IanB
author_rank: Super Contributor
timestamp: November 26, 2016, 12:24:25 am
content: I'm just giving you basic advice from my general knowledge of the subject. You must exercise your own due diligence about your design.That said, it is always advisable to include protection circuitry when using a lithium ion battery. You want to avoid overcharging, over discharging, and have short circuit protection. The short circuit current from such a battery can be very high.As for that particular chip, it seems to provide for a 500 mA charge current. Depending on the capacity of your battery that could lead to a very long charge time.I don't know about drawing power from the batteries while charging them. It might be OK, or it might confuse the charge controller. You might need to experiment, perhaps?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 26, 2016, 09:24:18 am
content: The problem in paralleling exists only at the moment you connect them in parallel. If they are at different voltages, a large current, only limited by the DC resistance of the cells, flows - the cell with higher initial voltage charges the another one. This current can be large enough to severely exceed the maximum allowed charge rate of the emptier cell. So you need some kind of safety logic before the actual parallel connection is established; or you need to do it by hand and never make a mistake. This can be cumbersome, because when the voltage differential is too big and you cannot connect them in parallel, you would need to charge one of them first separately.Otherwise, parallel charging is fine, as long as they are the same type, or similar; they don't need to be exactly in the same condition. The more they differ, the more you need to derate charging current, but you can't go wrong if you charge at the current which would be fine even for just one cell. I.e., if both are specified for 1A maximum charge, you can charge at 2A only if the cells match fairly well, but you can always charge at 1A even if they are mismatched in DCR and Capacity.But normally, you just charge both separately. This means using two separate chargers. No big deal, if you can make one charger, you can make a copy of your design. If you buy a charger, you can just buy two.If the cells are in series, it's a different issue altogether, but I'm assuming they are not in series because you didn't say so, and you are talking about paralleling them.You can parallel any two identical li-ion cells, and they act exactly as a single bigger cell. You should preferably use bare cells without any "protection" circuitry, and add the protection to the block of the two parallel cells. But user-removable parallel connections are a huge pain in the ass, so basically, when you parallel, you are making a new fixed bigger "cell" that will stay that way till the end.

author: tyguy2
author_rank: Unknown
timestamp: November 26, 2016, 03:40:09 pm
content: In regards to separate chargers, the charging circuitry for this is going to be embedded into a PCB, so removal of the batteries for charging is not going to be practical. If I buy these batteries and charge them to capacity, then parallel them, add protection circuitry to the pack, then I should be fine, correct?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 28, 2016, 08:04:17 am
content: Quote from: tyguy2 on November 26, 2016, 03:40:09 pmYes, that's exactly how it's done. Measure the voltage differential before making the connection. When you buy proper quality cells, they come up in the same state-of-charge, and you typically measure only a few millivolts of difference between the cells, so you can just parallel them as the first thing.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 28, 2016, 08:34:56 am
content: Lithium batteries catch fire because of excess current flowing internally. Due to damaged insulators inside, caused by physical or thermal trauma.Paralleling batteries have a significant downside that one failing cell creates a chain reaction. If one cell decides to no longer provide sufficient voltage (eg: out of life) there will be currents inside the battery pack. This will cause extreme heat since these currents are not limited. If this is enough to damage cell A, it becomes a short to cell B+n. Causing those to fail.Nice related gif:https://gifs.com/gif/Nx7pkK

author: Siwastaja
author_rank: Super Contributor
timestamp: November 28, 2016, 09:18:33 am
content: That's a commonly touted forum myth, but has no basis on reality.In theory, the chances that an internal short (or partial short, anyway causing excessive current draw from other parts of the battery) occurs in two paralleled cells is basically the same as the same fault occurring in a single cell twice as big (same amount of active surface area, same area of separator, etc.)In practice, smaller cells can have better quality control, better thermal properties (nearly isothermal construction), etc. Also, paralleling gives more freedom in the selection of the product you are going to use, especially if you need capacities over about 3-4 Ah.When paralleling, there are larger number of cell-wide safety devices, such as PTCs, CIDs or rupture valves; the units protected by these devices are smaller. This increases security: when one of the cells is failing and is equipped with safety devices, and is possibly at least somewhat physically separated from the other paralleled cells, the chances of the cascading failure are lower than if it was a single cell with no barriers between its internal parts.This is why paralleling tens of 18650 cells in EVs, such as Tesla, is not an issue. Laptop battery packs have always used several paralleled cells as well.The wrong assumption of the safety of parallel connection often causes amateurs to choose excessively big cells, only manufactured by shoddy and unqualified niche businesses. These products cause real fires.Of course, with some random low-cost pouch cells ("lipo"s), it's very unlikely that there is much added security from using two cells in parallel, because there is no per-cell safety containment and possibly no overcurrent/overtemperature devices per cell; also the isothermalness, which helps shutdown separators to do their job, doesn't help if two pouch cells are directly mounted together. All of this doesn't make it worse, however. In these cases, paralleled cells are exactly equivalent to using a bigger cell. Think of the internals being cut in two and moved into separate packaging, but electrically connected exactly like they are in a bigger cell.Similarly, a bigger cell is literally two smaller cells seamlessly connected in parallel inside the same casing. If a fault develops at one end, nothing is helping between the imaginary barrier between the cell halves. With two separate cells, you at least have chances of having/adding such barriers! Atworst, it's the same.In real life, good quality cells must have and do have rather good safety design integrated into the separators themselves, so that the faulty part of the cell is going to be "disabled", so it's not a cell-level thing at all.TLDR; li-ion cells don't have "brains" that decide something at "cell level". It's just laminated structure of electrodes sandwiched with separator, filled with electrolyte, put into a case (vacuum pouch, for example). Cutting this structure into multiple cases doesn't magically make faults propagate more badly than if they were happening inside one single case; in fact, the opposite is true.Is that .gif really related, i.e., showing a fault caused by paralleling of small cells, or is this just emotion-based (instead of facts) fearmongering argument typical of these discussions?

author: Jeroen3
author_rank: Super Contributor
timestamp: November 28, 2016, 12:26:09 pm
content: Quote from: Siwastaja on November 28, 2016, 09:18:33 amProbably. However, it is what happens when you wire them up without protection. RC model planes/cars often use unprotected series or parallel cells. And these areabused. Maximum possible currents, short bursts. Overloading. Expanding.It's fun, as a spectator.If you were to use hobbyking quality cells with your experiment. I'd reconsider.Quote from: Siwastaja on November 28, 2016, 09:18:33 amYes. I don't even know if it were lithium batteries. But it's similar to the outgassing of those 5Ah 3S packs.Looking that an MCP73811/2 was considered, which comes in SOT23, it isn't that large scale.

author: raptor1956
author_rank: Frequent Contributor
timestamp: November 28, 2016, 10:52:45 pm
content: You could charge all the cells to be put in parallel to the same voltage using a precision bench power supply to start with. Then, connect them together with current limiting resistors and let the fully equalize. Finally, when ready to connect in parallel disconnect the batteries using the resistors and connect as needed. There may still be a small potential difference, but when connected the internal resistance of the cells will compensate for whatever small voltage difference may exist and do so without drawing high current. Probably best to start with a cell voltage about 1/3 above the discharged level -- say about 3.5V for LiPo.Brian