Battery charging basics

Batteries, Chargers, and Battery Management Systems.
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rick_p   10 W

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Battery charging basics

Post by rick_p » Mar 29 2021 3:56pm

I looked through the FAQs on this site and did some general searching on the web. I found lots of threads with similar questions but not close enough to trust the answers are applicable to my situations. I have two basic questions.

1. Is the charger's ability to detect when the battery has reached full charge and stop charging the only difference between a charger designed to charge a battery versus one that was designed to provide constant power to a device? If so, does the battery's BMS prevent over charging if charger doesn't have the detection capability?

2. The next question I have is, I have a charger intended for use with a scooter (Lithium Ion battery) that puts out 42 volts 1 amp. It has the correct tip (fit and polarity) to fit a 48 volt (Lithium Ion battery) I have. What, if any, harm might come to the battery if I used this charger to charge it? If it is safe (harmless) to use it to charge the battery, I presume it will never fully charge the battery, is that in and of itself bad for the battery aside from any other concerns?

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Re: Battery charging basics

Post by amberwolf » Mar 29 2021 5:26pm

rick_p wrote:
Mar 29 2021 3:56pm
1. Is the charger's ability to detect when the battery has reached full charge and stop charging the only difference between a charger designed to charge a battery versus one that was designed to provide constant power to a device?
Mostly, yes, as long as "one that was designed to provide constant power to a device" means a CC/CV PSU (like the meanwell HLG or ELG LED PSUs, etc) that limits current by reducing voltage to maintain constant current, rather than one that limits current by hiccuping, or shutting off output until the overload goes away.

If "one that was designed to provide constant power to a device" means a random wallwart / AC adapter from some device, then no, those are unlikely to be CC/CV and won't safely charge a battery (some of them may not even *have* current limiting and will try to supply whatever is asked of them till they catch fire or otherwise fail).
If so, does the battery's BMS prevent over charging if charger doesn't have the detection capability?
It should...but don't rely on this, and make sure the full voltage of the charger or PSU is not enough more than the battery's full charge to allow it to ever overcharge. (don't use a 60v PSU on a 13s 48v battery ;) ) Same thing applies to chargers...if their voltage is too high, they won't shut off at "full charge" for the *battery* because they are trying to fill it up to what they're rated at, rather than what the *battery* needs. (they just detect that current drops below a certain point and turn off the output; that's all).
2. The next question I have is, I have a charger intended for use with a scooter (Lithium Ion battery) that puts out 42 volts 1 amp. It has the correct tip (fit and polarity) to fit a 48 volt (Lithium Ion battery) I have. What, if any, harm might come to the battery if I used this charger to charge it? If it is safe (harmless) to use it to charge the battery, I presume it will never fully charge the battery, is that in and of itself bad for the battery aside from any other concerns?
No direct harm...but it won't balance the battery, as that requires a voltage high enough to make the BMS start that process, which only typically happens near or at full charge. So if it's like most ebike or scooter batteries, it will get farther out of balance every time it is used and not fully charged.

Since 42v is pretty close to empty for a 48v battery, then it could get pretty far out of balance pretty quickly.

It also wont' have much capacity, and will perform like a mostly empty battery. If, for example, normally you'd get 10 miles out of it, at only 42v full charge it'll probably be a mile or less, and it wont' be able to provide as much power, and may even cut out above some throttle level.

Your controller may not even operate (turn on but not activate the motor) at that voltage, if it's a 48v controller and it has a high enough LVC.

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Re: Battery charging basics

Post by rick_p » Mar 29 2021 6:40pm

Those are the BEST answers ever. I sure hope search engines return this page for similarly searched questions because this answers my questions and a whole lot more. No, I can't use a wall wart charger that wasn't meant for charging batteries, it needs to be a charger that limits current by reducing voltage to maintain constant current. And although it won't damage the battery, charging a battery with a charger who's output is a lower voltage rating than that of the battery, is going to result in very poor performance, possibly to the point of being unusable if the cells get too far out of balance, even if the charger is a proper battery charger. Thank you so much for these replies!!

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Re: Battery charging basics

Post by rick_p » Apr 01 2021 9:33pm

I have another basic battery charging question. Do LiFePO4 batteries require a special charger? When I say special, what I mean is, can you use the same charger designed for Lithium Ion batteries, or is there a difference between the way the two chemistries need to be charged?

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Re: Battery charging basics

Post by rick_p » Apr 02 2021 1:39pm

rick_p wrote:
Mar 29 2021 3:56pm
The next question I have is, I have a charger intended for use with a scooter (Lithium Ion battery) that puts out 42 volts 1 amp. It has the correct tip (fit and polarity) to fit a 48 volt (Lithium Ion battery) I have. What, if any, harm might come to the battery if I used this charger to charge it? If it is safe (harmless) to use it to charge the battery, I presume it will never fully charge the battery, is that in and of itself bad for the battery aside from any other concerns?
amberwolf wrote:
Mar 29 2021 5:26pm
No direct harm...but it won't balance the battery, as that requires a voltage high enough to make the BMS start that process, which only typically happens near or at full charge. So if it's like most ebike or scooter batteries, it will get farther out of balance every time it is used and not fully charged.
The follow up question I have about a charging LiFePO4 batteries is somewhat related to the previous question about having a charger intended for use with a scooter that has a Lithium Ion battery because I recently picked up a new (pretty old but not very used) bike and I "might" need to order a charger for it depending on the answer to the LiFePO4 question.

The bike is an iZip E3VIBE that came with two batteries. The original owner sold it because they left it out in the rain with the charger plugged in, and needless to say, the charger fried. The on/off switch at the throttle didn't work either, so the bike appeared totally dead, batteries and all.

However, I think the batteries are OK though, I removed them from the bike and both had a little charge on them, so dismantled the throttle to have a look at the switch, I couldn't see anything obvious so I removed the switch and dismantled it too. I found corrosion inside but there was no sign of short-circuiting and the springs were fine, so I cleaned up the corrosion and reassembled everything, and sure enough, the bike turned on so I took it for a ride. The bike is super comfortable, I'm really impressed with it, it will be great for my wife to ride with me.

The batteries are 24V LiFePO4 batteries and by coincidence my daughter has a 24 volt scooter, which has a 24 volt 1.8 amp charger. The specs are almost identical to the specs of the fried original. However, the batteries have stickers on then that read "Requires special charger" so I'm afraid to use the scooter charger until I find out if LiFePO4 batteries require a different charger or not due to the chemistry difference?

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Re: Battery charging basics

Post by rick_p » Apr 04 2021 8:39pm

I’m going to answer my own question here because I found this post:

viewtopic.php?t=102134

Where someone asked a different question but in the responses my question is answered as well...

There is a difference between a charger designed for lithium ion batteries and one designed for LiFePO4 batteries. A charger designed for lithium ion might overcharge a LiFePO4 battery a little, which has a potential for danger but at the very least will shorten the life of the battery if used repeatedly.

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Re: Battery charging basics

Post by flippy » Apr 05 2021 4:15am

rick_p wrote:
Apr 04 2021 8:39pm
I’m going to answer my own question here because I found this post:

viewtopic.php?t=102134

Where someone asked a different question but in the responses my question is answered as well...

There is a difference between a charger designed for lithium ion batteries and one designed for LiFePO4 batteries. A charger designed for lithium ion might overcharge a LiFePO4 battery a little, which has a potential for danger but at the very least will shorten the life of the battery if used repeatedly.
thats just wrong. lithium chargers are not designed for any chemistry despite their marketing label. the only thing that matters is the set voltage. the charger has no clue about the chemistry nor does it care. that a certain voltage matches a certain battery chemistry is just for marketing purposes and for the less savy consumer to see if they have the right voltage charger.

charging SLA batteries and NIMH cells for example is actually tricky to charge well. lithium is stupidly simple by comparison.
Lithium beats liquid dinosaurs.

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Re: Battery charging basics

Post by rick_p » Apr 05 2021 10:13am

flippy wrote:
Apr 05 2021 4:15am
thats just wrong. lithium chargers are not designed for any chemistry despite their marketing label. the only thing that matters is the set voltage. the charger has no clue about the chemistry nor does it care. that a certain voltage matches a certain battery chemistry is just for marketing purposes and for the less savy consumer to see if they have the right voltage charger.

charging SLA batteries and NIMH cells for example is actually tricky to charge well. lithium is stupidly simple by comparison.
That explains a lot, and is excellent news because after reading that thread and then reposting what I understood from it here, I searched for the exact model [ hp0060w(l2) ] of the original charger, which I hadn’t done before, and I was thoroughly confused when I found that it has been mostly used/sold as a charger for electric wheelchairs, with no mention of the chemistry of the batteries, but oftentimes it was stated to be a lithium ion charger. The replacement chargers were ridiculously expensive, I’m guessing just because they can get away with it due to it being for a wheelchair. I wasn’t able to find a single reference of it being used/sold for an ebike at all, let alone mine, which I thought was odd, I guess this bike wasn’t very popular.

At any rate, since the only thing that matters is the set voltage, then finding a suitable aftermarket charger ought to be easy, there are lots of reasonably priced 24 volt 2amp chargers that have the correct XLR connector with the correct polarity on the pins.

I do have two questions though, according to the label, my charger’s set voltage is DC OUTPUT: +24V 2A
Some chargers I see for sale have the exact same specs, but underneath that it also has: CHARGING: 29.4V MAX
Mine doesn’t have that, should I avoid a charger that says that?

And lastly, what actual voltage reading should I expect to get from my 24 volt battery after it is finished charging?

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Re: Battery charging basics

Post by flippy » Apr 05 2021 10:25am

see, this is what makes everything comfusing for everybody. marketing over standards.

ignore the "marketed" voltage and actually look at the numbers on the battery.

a regular li-ion battery at "24V" means its usually a 7S battery pack (7*3.6=25.2V) and peak (fully charged) is 7*4.2=29.4V
but as you see the NOMINAL voltage is NOT 24V, its 25.2V. wich makes it a LOT more confusing because if you have lifepo4 cells it would be 8*3.2=25.6V NOMINAL with a fully charged voltage of 8*3.65=29.2V.

see how quickly and confusing it can get if manufacturers start obvuscating what you actually have? just look at power tools and the half dozen ways they advertise a simple 18V 5S battery, you will see every voltage between 18~21V on the makreting and printed on the batteries just to have 1 more then the competition, because 1 mroe is better, despite it being the exact same 5S battery setup.

ps: a mean well SP-100-27 would suit your battery well. you can just cut the wire of the 48V charger you already have and hook it up to the terminal. set the correct voltage and you are off. and SP-75-27 would also do the trick if you dont need to charge as fast. slower = better.
Lithium beats liquid dinosaurs.

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Re: Battery charging basics

Post by rick_p » Apr 05 2021 6:32pm

flippy wrote:
Apr 05 2021 10:25am
If you have lifepo4 cells it would be 8*3.2=25.6V NOMINAL with a fully charged voltage of 8*3.65=29.2V.
I do have a LiFePO4 battery, I will check the fully charged voltage as soon as I get my hands on a charger.
flippy wrote:
Apr 05 2021 10:25am
ps: a mean well SP-100-27 would suit your battery well. you can just cut the wire of the 48V charger you already have and hook it up to the terminal. set the correct voltage and you are off. and SP-75-27 would also do the trick if you dont need to charge as fast. slower = better.
Thanks for the recommendation, I really appreciate that. However, this isn't my only bike and those are a little on the expensive side for this particular bike. I'll probably buy one of those that are similar in nature to the one the bike came with. Hence the reason I asked the following question, which I would very much appreciate your advice on...

According to the label on my charger, it has a set voltage of DC OUTPUT: +24V 2A
Some chargers I see have the same exact output specs, but underneath that it also has: CHARGING: 29.4V MAX
A charger like that has a max charging output of .2V higher than what my battery should hold when fully charged, is that OK to use?

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Re: Battery charging basics

Post by flippy » Apr 06 2021 3:26am

For lifepo4 you can slighty overcharge them if the current is low enough. So yes.
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Re: Battery charging basics

Post by Barncat » Apr 08 2021 9:20am

I have a related question. For a battery without BMS of nominal 3.6v li-ion cells and a generic bulk charger, when does the taper to the CV phase generally start? Very near the fully charged 4.2v/cell, or sooner? As flippy stated above the charger has "no clue" what it's connected to, and is just a fixed watt generator in the CC phase?

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Re: Battery charging basics

Post by flippy » Apr 08 2021 9:24am

Barncat wrote:
Apr 08 2021 9:20am
I have a related question. For a battery without BMS of nominal 3.6v li-ion cells and a generic bulk charger, when does the taper to the CV phase generally start? Very near the fully charged 4.2v/cell, or sooner? As flippy stated above the charger has "no clue" what it's connected to, and is just a fixed watt generator in the CC phase?
that is impossible to say. if you charge with 50mA you will never see a CV stage.

the harder and faster you charge the sooner you hit CV. many high power DC chargers for cars will hit CV well into the low 70% range.

that there is a bms or not has no impact on the charging process. the bms only does something if somethig goes wrong. its a protection device, not a control device. it can only stop/pause the charging by simply cutting the input off, it does not control it. it simply shuts the door.
Lithium beats liquid dinosaurs.

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Re: Battery charging basics

Post by spinningmagnets » Apr 08 2021 10:07am

For a few years, one manufacturer called 14S lithium-Ion battery packs 50V. After the Bafang mid drives began growing in popularity, they were often used with 14S, and many began labeling the packs 52V. EGO-brand lawn tools call 14S 56V, and I think Echo calls them 58V. So which is it, and what charger can charge them?

The specs from the manufacturer say that each cell can be charged to 4.2V per cell maximum. Later, we found out that the pack life can be doubled (or more) if charged to only 4.1V per cell. The nominal "average" voltage from full to empty is 3.7V, and they really shouldn't be discharged below 3.0V, with 3.3V being a good target. Sooo...3.3V to 4.1V per cell.

For 14S, that would be setting the low voltage cutoff at 46.2V, and the pack would be fully charged at 57.4V (*nominal average is 51.8V).

LiFePO4 is a nominal 3.2V (*instead of 3.7V). I've read it can be charged to 3.65V per cell, but as we learned before, do not store lithium at max volts, therefore running LiFePO4 in the flat middle of its curve would be 3.0V-3.4V

When a lithium-Ion or a LiFePO4 pack are advertised as a "12V replacement", they are two different voltages. That's why they need a special charger, not because if the chemistry.

3S Lion is_______12.3V
4S LiFePO4 is___13.6V
4S LIon is_______16.4V

My car jump-starter box is 16V max (4S Lion), no BMS. The alternator only goes as high as 14V, so that's it's voltage when I use it.

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Re: Battery charging basics

Post by rick_p » Apr 08 2021 6:40pm

This post is not necessarily related to the "related question" by Barncat or the replies from flippy or spinningmagnets, I'm just reporting back the results of charging my "24 volt" LiFePO4 batteries with a 24 volt 1.8 amp scooter charger.

Both batteries charged to 27 volts, give or take a small fraction of a volt. I assume this is when the charger detected the batteries had reached full charge. The charger got a little warmer than I might have expected, but it's possible I was just paying closer attention to it than I normally would.

I'm adding the following pictures and information about the batteries just in case it adds anything to the conversation.

One of the two batteries doesn't have a specification sticker on it, so the Ah is unknown but it is marked 24 volts next to the charging port. It only has one pack inside it, which appears to have 12 cells total.
small-battery.jpg
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A close up of one cell.
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I found it odd that there is no fuse in this battery, even though the case clearly came designed to have one.
small-battery-no-fuse.jpg
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The other of the two batteries has a specification sticker on it, and says that it's rated at 9.6 Ah. It has two packs inside and it appears that there are 12 cells in each pack.
large-battery.jpg
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A close up of one cell.
large-battery-cell.jpg
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A close up of the sticker. I find it interesting that on the line where is gives the model, it has (RMB-Li RealForce), which left me wondering if that stood for lithium ion, but I looked it up and the cells are definitely LiFePO4.
large-battery-sticker.jpg
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