Li-Ion questions (charger, fast-cycling)

Most of what I know comes from playing with model airplanes, so take this all with grains-o-salt:

On a charger that goes to 10.2V -- Eeek. Not on a model airplane you don't! Because model airplanes don't come with any of those wussy protection circuits -- people buy ammo boxes and fiberglass bags for battery charging for a REASON, and they post YouTube movies and RC Groups pictures of burning airplanes for roughly the same reason.

I suppose that if it's all part of a system designed for it it's OK. Just don't mix and match. Is this some anonymous Chinese company that sells stuff, that you'll never be able to sue if your house burns down, or is it a good old 'merican company with a good old 'merican address to which writs may be sent?

If you want "safe" charging and aren't interested in learning how to design LiPo chargers, buy one designed for model airplanes off of Tower Hobbies (expensive but sue-able) or Hobby King (front-men for un-sue-able Chinese). Get a balancing charger.

It could be that the battery pack has its own built-in balancing charger, and the 10.2V is just a more-or-less raw supply.

As to charge/discharge: I had not heard about LiPo's not liking rapid charge/discharge, but then, you don't do that much with model airplanes. I _do_ know that LiPo's don't like heat, and like any other battery they get hot with high-current use. They don't like to charge as fast as they discharge.

What REALLY makes a LiPo pack unhappy is lots of partial charge/discharge cycles without a balancing charger -- LiPo cells are very sensitive both to over charge and under charge, and a pack that's subject to a lot of partial cycles tends to get one cell that drifts toward too much charge and the other to too little. Modern chargers monitor each cell voltage and correct for this. If'n you're going to go there, I suggest using a charger that balances the cells, and something that protects from under- voltage (even if it means having the dynamo come on when you're huffing up a hill).

HTH.

Oh -- watch the pack for puffing; if it does put it someplace where it won't catch things on fire when it bursts into flame. The proper way to discard a suspicious pack, in order of the distance you live from the authorities, is to (1) toss it it salt water for 24 hours, then toss it in the trash; (2) pound a screwdriver through the center of the pack, let it burn off, then throw the remains away; (3) duct-tape it to a fence post and shoot at it until it burns, then proceed as (2).

It is from a seller in New York, fulfilled by Amazon. All comes from China, of course.

Balancing would require hacking the battery packs for the middle contact, somthing I'd like to avoid.

If does but at the same time that is also the defense of last resort. One of the batteries inadvertently opened when the rear tire on my mountain bike blew and sent some pieces scattering. There was a circuit board dangling of of the now disrobed battery pack.

It's not Li-Po like the RC guys use, it's Li-Ion like in laptop batteries. The cells used in manufacturing could sometimes actually be scavenged laptop cells. Nobody really knows but they do work nicely.

I think balancing is done at the pack. But probably by crude shunting as I did not see any inductors. The size would be selected large enough so it'll never deplete during a ride, even one where I have to throttle down for slowpokes.

It just puzzled me when I read that Li-Ion doesn't like short-cycling.

:-)

Not exactly the same, but I've seen a similar configuration in a Chinese MP3-playing toy. The configuration looked like:

USB port -> 10 Ohm series resistor -> silicon diode -> LiPo battry.

No other relevant components in the direct power path on the main board, but the battery did have a small protection board, apparently with the classic one chip and 2 transistor solution that seems to be common nowadays.

The device in question did have an "interesting" quirk: When fully charged, it would fail to operate when the charger was disconnected. Switching it off and on with a (mechanical) switch would reset it.

Apparently they indeed relied on the protection circuit to work as a simple ON/OFF charge controller. It would turn off on overvoltage and stay that way, so that when the charger was disconnected, the sharp voltage drop to zero (before the the protection has time to sense that the overload has gone away) would upset the main ASIC, which would get stuck somewhere in the wilderness of an improperly timed reset cycle.

As for safety: There's a good reason to have control and protection circuits separate. A single fault condition can't normally kill both together (unless the protection is way inadequate). With only one power interrupting component (one MOSFET per direction of charge / discharge current flow) all it takes is a single fault in a semiconductor part to deadlock this thing into an always on charging condition, with a third party uncontrolled tolerance "nominally 5V" adapter for the power.

Dimitrij

Probably because the battery's OVP had come and needed to be "unstuck".

A stuck ASIC would be poor design.

Single-fault safety is why I don't like the scheme where they use a higher external voltage. I can build a supply with exactly 8.2V but I tried that and it has one major disadvantage: As the battery voltage approaches 8.2V the charge current will become very small due to cable resistance. It'll take all day to recharge a battery to a reasonable level. That would present a minor inconvencience if I did a long ride late into the evening and wanted to head out again in the morning. For safety I ride with bright daytime lights.

A scheme that would work but I haven't seen it implemented is this: Apply a current limited 10-12V externally and have the control electronics in the supply disconnect it every few seconds. Check open circuit voltage of the connected Li-Ion battery, if less than target voltage apply another burst. If at target voltage stop and signal a green LED for "charge is complete".

If I wanted to do this right it'll be another electronics project and my hobby time is already occupied with bicycling and beer brewing. Maybe with a uC but as an analog guys I am not a great programmer. So possibly I'll do it analog.

"proper" charging of a LiPo is 4.2V/cell (or whatever the pack calls for) or some maximum current, whichever is lower. Much like lead-acid, except with more potential for flame.

The pack _should_ charge at maximum current until it hits 4.2V/cell, then stay there with the battery determining the current, then finally the charge should terminate when the current drops to some fraction of the cell's maximum current (IIRC 1/10th is the rule of thumb, but don't go designing a charger based on that).

Ideally yes but for expedient charging this requires sense lines to the battery pack which, on a bicycle, you don't have.

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can't just use a four wire cable leaving only the wire from connector to cells (which I'd expect to be very short) unaccounted for?

-Lasse

It voids any warranty and liability protection if you cut open the enclosure of the battery pack. There are no screws, it has to be cut and pried away.

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but won't the wiring from whatever connector is on the battery pack to the cells inside the pack be very limited?

-Lasse

It is generally a round cable with a standard connector. Looks like this:

The copper inside isn't very thick, meaning it'll easily drop 100mV. Same for the charger cable. During discharge it doesn't matter much because good bicycle lights contain a buck converter. In my case that keeps the light level the same all the way down to

is there a separate cable/connector for charging and "discharging"

-Lasse

No, same cable. But on my mountain bike I mounted this fairly heavy battery in a sturdy box, screwed down via a bracket but cushioned with rubber. Added a fuse and a switch. Two switched cables come out for front and back light plus a non-switched one for charging or connecting miscellaneous loads. I left the original cable on there, coiled up and strapped in. This has withstood hard rock hits.

He's a user, not a maker in this case.

Really, the charging circuitry should be co-located with the battery.

(snip)

That is what the charge regime was for the MAX1737 when I used it waaaay back (2002 actually, just checked) in a commercial Li-Ion charger. After a pre-qualification phase (ensuring battery voltage above LVCO and temp within limits), current-limited constant-voltage, terminating when current drops to 10% and a saftey timeout.