It seems that there is a great deal about batteries that I do not know - so I am asking before I ruin something.
I have a electronic device that has a 7.4V 2400mAh Li-ion battery attached. The battery is obviously only used when the power goes out to keep the RAM alive and back it up to the attached CF card.
The device is an ACARD-9010 Ram Disk
Anyway the battery is clearly knackered (and I have had it for a long time) and getting a replacement is proving a bit difficult.
So I thought I would buy a 6 cell AAA or AA adapter (in series) - depending on what I can get. Attach the appropriate connector to it (probably cut from the existing unit) and plug it in.
6*1.2V = 7.4 so that’s right
But 6*850mAh AAA cells (approx but measured) = 5100mAh which is substantially greater than the 2400 of the original battery. 6 * AA would be even larger
Am I barking up the wrong tree here - or are things as simple as they appear to be.
Looks like 2x18650
1 18650 has a voltage of 3.7V (actually between 3.0 and 4.2 depening on charge state, but is classified as 3.7)
2 of those make 7.4v. Size of the pack looks about 2x18650
and 2400mAh 18650 is nothing special.
And when you connect batteries in series, like you do with adding the voltage for the aaa’s, you can’t add the mAh.
It’s either parallel and add the capacity, or in series and add the voltage.
So 2x 18650 2400mAh in series makes 7.4v 2400maH
2x 18650 in parallel would make 3.7V 4800mAh
Sorry for the caps but I need to reiterate, DONT DO THAT!
Just because the voltage is the same the charging for a Li-ion pack is way different from charging for a NiMH pack. Your only option for a rebuild (and this would really be last resort) would be to use 2 Li-ion’s in series, thats the ONLY way to rebuild that pack.
I cant gauge size from the stock photo but I imagine there are either 18650’s or 14500’s inside that pack, here in the US there are several options for have a Li-ion pack professionally rebuilt, have you looked into that? It is possible to do it yourself but (and please dont take this the wrong way) asking a question such as this tells me you just dont know enough to safely do it, not that you couldn’t learn and eventually do it but at this time I’d highly advise against it. Li-ion batteries are much much more dangerous that NiMH, you could have a catastrophic failure with NiMH and it wouldn’t be that violent, a catastrophic failure with Li-ion is an explosion, not a little pop and some smoke, literally an explosion.
2 * Ebay Link
[No comment on quality - its just a convenient URL]
so unwrap the existing pack, cut out the existing cells, replace, rewrap and “Bobs my Uncle”
Is the battery pack just two batteries plus wrapping, or is there likely to be anything else in there (I have never opened one up) like some sort of circuit?
Any thoughts anyone? Or am I really barking up the wrong tree. I don’t seem to be able to buy a premade pack anywhere for anything remotely like sensible money
DOH! Forgot about the specialized charging pattern needed…yeah…if it’s charging NiMH for the backup and you try to put LiIon in it’s place without charger controlled charging…bad things can happen…duh
Thanks Cereal…
If it came with a 7.4vdc Li Ion pack and you just want to build your own (because it already has charge controller charging) then the 2S 18650 would work…
Is that picture in the OP the actual picture of the battery pack?
The IO rate is good (from memory, when I last measured it). However the major problem is size - 8*8GB DDR2 is now and always has been unaffordable so I use 8*4GB=32GB. A 32GB disk is less than entirely useful as serious storage
So what use is it. Well my PC is entirely SSD based so to avoid wear cycles I use the RAM disk as the swap drive and windows temp directory. That way it doesn’t matter if I lose what’s on it (but it is a nusiance to lose what’s there as it needs reconfiguring and the reason I want the battery) - and I don’t use SSD space for crap. You can split it into two banks and run it as RAID 0 which allegedly doubles the IO for no loss of disk space - but I have never done that. Never got around to it
Think database I/O cycles…imagine a web server that generates thousands of search read/write cycles a second…that is where the speed of this type drive really comes into play…but using it as a SWAP space to save your solid state drives is ingenious however…
Have you stripped the outer heat shrinking off the old battery to see what kind of board is in there for load balancing and whatnot?
Like comfychair said you might be able to rip the old batteries out and solder in new batts and voila…no need to change anything just replace the defective cells
I have taken the pack apart now and it does as suggested take 2 * 18650 dated 090792 with what looks like a model KFPFK37
A Number of things are clear.
This pack is not going back together - it was clearly never intended to come apart
There are three wires - red, black and white (central)
Red & Black appear to be +ve/-ve.
The white wire goes to what MIGHT be a temp sensor on a thin piece of wire which was embedded between the batteries and then connects back to the ve terminal Probably. The wire is delicate and broken - and I don’t THINK I broke it
Just a simple thermistor. Repair the wire. Who knows - maybe the cells themselves are fine, but the inop sensor was telling the charge circuitry there was a problem. Might be worth a try before you replace the cells.
edit: Is that a circuit board on the end with the wiring?
Also too, measure the voltage of each cell and tell us what it's at currently (you don't have to break the cells apart to do that). That pack isn't set up for any kind of balance charging, it would need a 4th wire running down to the tab on the R/H end that ties the cells together for the charge circuit to be able to do that. It just depends on luck for keeping the cells in the 'safe' voltage range. Over time the balance between the cells can get screwed up which shortens their life even more.
