Lithium polymer batteries.

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Lostgallifreyan
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Joined: Fri Jun 11, 2021 2:52 pm

Re: Lithium polymer batteries.

Post by Lostgallifreyan »

I looked for pictures of innards and saw that there are lots of different types now. Lots of demands for different support too. Can't ever get a consistent spec for a lithium PP3 that way, so it's a minefield.

We have to narrow the field a LOT, but the main base is the voltage of two SERIES cells, for an Organiser. Boost circuits waste, period, end of story. Not only are they impossible to predict outcomes for, the generation of 9V just wastes heat in the Organiser voltage regulator. So long as we don't insist on PP3 always meaning 9V we'll get by. :)

The only real risk we have is the lack of charge balancing in the series cells, but a lack of three terminals doesn't prove it's not there because an internal circuit can be designed to do it, we just can't know if there's an internal third wire to the junction of cells and the charge circuit without dismantling a battery. If there is one, then balancing would be what it's for, there'd be little point otherwise except perhaps to block input current entirely if it the circuit was just measuring, and didn't like what it saw.

There's so much complexity in choices now, that dangerous and misleading stuff is going with it. One claim is that the EBL batteries need a special 9V charger. They don't. The internal circuit limits charge voltage to a safe value based on any general purpose PP3 charger intended originally for nickel-based rechargeables. Some of EBL's own chargers make this clear, so why the claim exists at all I don't know. A future modification might make the internal circuit balance the charge on both cells, and that would be the ideal battery for an Organiser.

I saw a USB-based charge connection in the base of a lithium PP3 in one picture, but I don't see that as an advantage at all, it's more expensive, risks someone trying to charge it while the main terminals are still connected to a load, and a USB connector is a very weak method for something that regularly needs plugging for a current flow of many tens of milliamps or more. I like those EBL 600mAH batteries because they're cheap (about 3 quid each, easily comparable to a Duracell which they easily beat) , and they're easy to manage on a standard PP3 charger with no special considerations other than to charge them only when there's someone nearby and awake to be safe, and ideally to test the voltage after charge stops to see if there's any sign it might be failing. These are basic tests we might expect to have to make for any rechargeable. The more exotic the design, the harder the maintenance will be.

Re monitoring the charge current, a standard protection circuit would do that, but more specifically limit the current to what is best for the cells that come with it. This is more important than letting us see the actual value. The people who really know the specs are those who put the thing together. (We hope! :) They will have matched the protection circuit to the cells and made sure that standard chargers CAN work safely, not least because people will try that, it's just plain Sod's Law that they will, so you can bet that to pass underwriters lab tests to get the right to sell in the UK, the makers will do this. Our main concern might be that their rejects might get hauled out of a bin in China, relabelled, rewrapped, and sold as some other *Fire 'brand' and be bought by people who aren't aware of this kind of counterfeit until it burns their house down.

THAT little thought might be all we really need to make us want to be eternally vigilant. This is how I first approached those EBL batteries, but so far they seem to be solid. I've been using them for about two years. I've never actually had a lithium cell of any kind get hot and burn or explode. I just watch them on general principle because it's easier than cleaning up if I didn't and it went bad.

PS. Thanks for that info about the Organiser watching the 5V line, I didn't know that. We should still get adequate warning though, because even though there's a low internal resistance in the lithium batteries, there's a voltage drop in the regulator that will always cause the 5V line to drop enough to cause the warning before the real power loss is dangerous. Alkalines are risky because when they get low the internal resistance is too high to allow safety for data, even if it's on an attached RAM pack. The simple act of plugging a top-slot adapter might cause a spike that can kill data in RAM at that point. If it's already powered the risk is lower, but I think the manual says we shouldn't be doing that either. I think the safest way is always to use the best internal battery we can get, and we now have a choice that is better than anything we had to use before.
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