18650 batteries in series & parallel- how to calculate charge voltage?

[Kruck]

Hi all-

I am preparing to convert an 18v NiCad tool to Li-Ion (my first conversion!). I've got some new high-discharge Li-Ion 18650 batteries, access to a battery spot welder, and a 5-cell BMS board.

If all goes well, I will have 5 cells in series (~18v) and if I have room, a second series of 5 in parallel to reduce the draw by splitting it across the two series of cells.

So here's the question- I want to include a voltage converter in my design so I can use the original battery charger, but converted to the correct voltage. I have two original chargers, I think one is 21v and one is 24, both are very low amperage. I'm thinking a DC-DC converter to get the voltage into the correct range, I don't mind if it trickle charges for a long time to reach charge, I don't use these tools often.

So: If a single cell charges at 4.2, I'm assuming the BMS is (in essence) charging each cell independently rather than the series of 5 cells, so I should get a converter that gets me to 4.2, is that correct?

Are there any *risks* to charging this slowly (low amperage)? For example, will a standard BMS think that the charging is so slow it will shut itself off? Will too-slow charging have any negative impact on the batteries themselves?

Many thanks!

 

[jay32267]

I'm looking at this BMS and I don't see how it could charge independently with that many leads.

https://www.ebay.com/p/18v-5s-15a-5-Cell-Li-ion-Lithium-Drill-Battery-BMS-Protection-PCB-Circuit-Board/809615325?iid=273253323273&chn=ps

 

[Kruck]

I'm looking at this BMS and I don't see how it could charge independently with that many leads.

https://www.ebay.com/p/18v-5s-15a-5-Cell-Li-ion-Lithium-Drill-Battery-BMS-Protection-PCB-Circuit-Board/809615325?iid=273253323273&chn=ps

Different look, but that is probably effectively the same as what I ordered:
https://www.ebay.com/itm/5S-15A-Li-ion-Lithium-Battery-BMS-18650-Charger-Protection-Board-18V-21V-Cell/302768845479
http://

So I should be using 4.2v x 5 = 21v as the input, which makes sense per the ebay link above (where I ordered it, the charging voltage wasn't provided, but on the ebay post it does say 21v)

So, I can use my 21v without adjustment, and I'd have to use a DC voltage converter if I wanted to use the other charger to go from 24 ->21 v... but the converter will have to live inside the battery pack, which leaves me with question 2 below

I've still got question 1 leftover from the original post; (1) are there any issues with charging 18650s at extremely low amperage (I think both chargers are less than 1 amp total, across either 5 or 10 18650 cells)- will that cause BMS to cut off charging because not enough current is coming in? Or any risk to the cells themselves?

(2) if I buy an adjustable voltage converter, and adjust it when hooked to the 24v supply to pass through 21 v... that converter would actually live inside the battery packs. If I want the option of charging both packs at once (one on each charger), would the voltage converter hooked to the 21v just pass through the whole 21v because that is what the output was adjusted to? Or is the stepdown /relative/ to the input, where the converter would then take the 21v input and convert it to something slightly lower?

Many thanks!

 

[jay32267]

"So I should be using 4.2v x 5 = 21v as the input"
Yes

"are there any issues with charging 18650s at extremely low amperage "
No

"I think both chargers are less than 1 amp total"
I think this is actually too much....i think you want to charge at around 200-400 mA from what I recall.

"Or is the stepdown /relative/ to the input"
depends on the converter....but usually it's not relative.