Converting old cordless tools to new lithium batteries.

Joe Public isn't supposed to be able to buy NiCd cells from "shops" now, but I doubt you have to look much further than RS to get them ...

Oh, they are still widely available. They have several advantages

- They don't catch fire like Lithiums

- They don't die from self discharge like NiMh - can always be recharged.

- They are in general tough and can take abuse

So are all there in the 'trade' market

Joe public is more likely to be sold the 18650 LiFePo cells, They are tough, and in many ways better all round for amateur use.

AFAIK they are not available as "batteries", but they are as cells for repairs. So either tagged, or in a cardboard cover for building packs, or a plain wrapper with dot-matrix specs printed on, but not colorful shiny labels.

Thomas Prufer

I think some still come with shiny labels.

But otherwise I think you are 100% correct

On 21/08/2025 11:28, The Natural Philosopher wrote: <snip>

Drop not rise?

Andy

ISTR that the voltage drop is proxy for the cell gets warm, which is proxy for the electricity can't be stored any more, i.e. the cell is full.

Thomas Prufer

Yes.

It's called 'delta peak' charger. If you ever had an RC car you would have had one.

Ive still got one left over. In general you can recharge from flat in around 20 minutes

Although the nickel cells get pretty warm!

I am not sure its the temperature that causes the drop in voltage though.

Well I am not sure about that, but it certainly is a reliable indicator of a fully charged NicCd or NiMh battery

I have a lithium charger that goes to 4,2V per cell, then stops and monitors the voltage off-charge. If its below 4.2V it puts another couple of minutes of charge in, repeating till the pack reliably holds

Most article are a bit vague in this area. I read one claiming the voltage drop was due to internal pressure due to the skew in preferential chemical reactions.

YMMV

Three terminal cells have a thermistor to sense the temperature rise.

Probably much better than "charge until it gets so hot that the resistance changes".

The RC car world has been using delta peak since I fisrt used NiCd on a plane - around 2000. There isn't a 'better way' or they would have used it.

So... the NiCads are on their way. I've had to buy 16 instead of the

..I don't think 7% over voltage is going to harm the hedge trimmer but I wonder about use of the charger? It's got a rated 23v output to charge an 18v battery pack - will there be enough potential difference to fully charge the newly made up 19,2v pack? Will it simply take longer to reach full charge or will it never quite get there?

Thanks,

Nick

Hmm. OK...

Mmm. I make a fully charged 16 cell at around 23V...may have some trouble getting full whack in.

If the pack is simply bare cells you can get a better charger

Especially if there's e.g. 0.6V drop from any reverse polarity protection?

So, rather than shell out for a new PSU (I don't have a 24v one in the spares box) it would be better to load the battery pack with 15 cells and put the spare one aside and only have to buy 14 cells next time[1].

Thanks,

Nick [1]Except by then I'll probably have forgotten where I put the spare cell and have to buy another 16 again.

Probably. :-)

Recelling the battery pack (15 not 16 cells) worked a treat. My only complaint is that these cells store about 35% more energy than the old cells so I have to cut the hedge for about 35% more time before I can loudly declare "Oh, the battery is flat" and bugger off and get a beer.

Thanks for all the advice.

Nick

:-)

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