Old NiCd drill battery - can I convert to Li-ion? (1st new thread since crash)

I have a Black & Decker Firestorm 14.4V drill & also a reciprocating saw and search light. The drill came with a battery and so did the saw/light and I also bought a spare. So I have 2 chargers & 3 batteries to go with the 3 tools. The drill & saw & light all work well and I'm too poor to replace them, so I want to see what I can do with what I have.


The batteries are 12 cell Sub-C NiCd

One of the batteries has been rebuilt (12 new cells) a couple of years back and is working well.

One of the batteries is original and reads 6.3V after charging - I've pulled this apart and 5 cells read over 1.2V and 7 cells read 0.00V.

The 3rd battery is also original and reads 9.4V after charging - I haven't pulled this one apart yet.


So - what to do? The rebuilt pack cost heaps to have rebuilt - around a hundred bucks! Stuff that - I'm not going to pay that for my other 2!


What I'm thinking:
Why don't I buy a hobby LiPo pack that will fit inside the battery housing and make that sucker work? A 4S LiPo pack with balance taps that should fit OK would cost around $16 + shipping from Hong Kong. 1600mAh is probably higher than my original sub-C NiCd cells and 30C constant/40C peak is probably enough output to run the drill or saw just fine.


So, has anyone tried this? Any reason why this wouldn't work?


Edit: For anyone wanting to do this it is important to realise that the hobby LiPo packs are unprotected LiCo batteries and there is some risk of a 'vent with flame' situation. There is no protection from the cells being discharged below the minimum recommended voltage, so adding a Low Battery Alarm to the pack would be a really good idea. Charging on a non-flammable surface would also be a sensible thing.

Oh yeah, I realise that my 2 chargers aren't going to be used to charge a battery converted to Li-ion cells, I've got 2 hobby chargers to do the charging once I convert my batteries.

Where you getting that 30C constant discharge?

srfreddy said:

Where you getting that 30C constant discharge?

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Well, that's what the specs on HobbyKing list - 30C constant and 40C Peak (10sec). I have found that my drill & saw run for well over 10 minutes of hard use so they only average somewhere below 6C. With lighter use they are probably under 2C average discharge.

My understanding is that high current drain isn't unusual on some RC planes, so the batteries have to be able to handle well over 10C as a matter of course. When I used to race RC cars it wasn't unusual to play with motors that would struggle to achieve 4 minutes run time which equals 15C average and a hell of a lot more than that for peak draw. (RC cars have a far less constant power draw than RC planes)

Of course if you regularly drain these batteries at 30C then the life span may not be all that long, but as long as the batteries can handle the needs of my drill then I'm happy.

The problem you will have is charging non nicad batteries properly (termination). I would more recommend buying some subC nicads and some flux, solder and a 100+watt soldering iron and some wire or metal strips that can handle current and solder up a pack of your own for cheaper.

Lynx_Arc said:

The problem you will have is charging non nicad batteries properly (termination). I would more recommend buying some subC nicads and some flux, solder and a 100+watt soldering iron and some wire or metal strips that can handle current and solder up a pack of your own for cheaper.

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I can't see how charging a 4S LiPo battery would be hard, I'm pretty sure that is the sort of battery that my hobby chargers were primarily designed to charge. These LiPo packs also have a balance lead with the right connector for my charger so that I can ensure all 4 cells CC/CV charge to 4.20V each time.

I can't see sub-C nicads being any cheaper either - I would need 12 cells instead of one 4S LiPo pack, the LiPo costs $16 + shipping. The 12 NiCd cells would weigh noticeably more so the shipping would be dearer too.

According to the specs on the LiPo battery it can peak for 10 seconds at 40C - that would equate to >900W of power. I'm pretty sure that my drill isn't capable of that much power, so I'm having trouble seeing why this wouldn't work.

Here's what I'd look into: What is going to happen when someone other than yourself puts your "modified" pack on the original NiCd charger? You would need to be sure this never happens even accidentally. How?

Otherwise, since the original packs are already bad, and you already have an appropriate hobby charger and know how to use it, it seems to my ignorant self that you don't have much to lose in trying your experiment. Let us know how it goes.

Sounds like a great idea to me, especially if you can incorporate one of those battery alarms or something to avoid over discharge.

Belker said:

Here's what I'd look into: What is going to happen when someone other than yourself puts your "modified" pack on the original NiCd charger? You would need to be sure this never happens even accidentally. How?

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Well all I can say is that just doesn't happen, no one goes using my stuff without asking my permission.

adam83 said:

Sounds like a great idea to me, especially if you can incorporate one of those battery alarms or something to avoid over discharge.

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Ahh, there's something that I haven't thought much about - over discharge. I was thinking that as soon as I notice power dropping/tool slowing then I'd swap batteries. These low battery alarms seem to be quite cheap and I think that I could easily fit one in beside the battery - this is probably worth doing.

Thanks for the suggestion.

OK, so now I have ordered the 2 x 1600mAh LiPo battery packs ($15.99ea) from HobbyKing and 2 x 4S low battery alarms from Ebay ($3.65ea, free shipping). Once these arrive I'll be able to convert the 2 dud drill batteries and hopefully the re-vamped batteries will run the drill & the reciprocating saw as good as the NiCd pack did.

I believe your new packs will work out great. Hopefully I will do the same one day with a B&D Firestorm 24v pack that crapped out and replace it with a 6s. When you are building your new packs measure the parasitic drain of the battery alarm so you have an idea of how long you could store the packs if they weren't being used frequently. Good luck with your retrofit and let us know how they work out!

adam83 said:

When you are building your new packs measure the parasitic drain of the battery alarm so you have an idea of how long you could store the packs if they weren't being used frequently.

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Another good suggestion - I'll measure the mA or uA and guesstimate how often that I should ensure I top up the batteries. I'll also make a note of when I charge them and then test them after a few weeks/months to see how they are doing.

I might look at whether it would be viable to unplug the battery alarm for storage, but if that won't really work out I'll just make a point of topping up the charge every now & then.

You could put a momentary switch on the packs such that the alarm is disonnected when the pack is not connected to the drill itself so extra packs would not have any self discharge issues from the alarm. You could then push the switch to "test" batteries if they are good and a beep means charge.

So far I have my LBAs (low battery alarms) ($3.65ea with worldwide free shipping) and am now waiting for my 4S LiPo packs to arrive. I have measured the current drain on one of the LBAs and it is 18.5mA - I definitely don't want to leave that connected permanently. What I am thinking of is to have the LBA accessible and able to be unplugged when not needed. The LBA will plug right onto the balance plug of the LiPo, so I can unplug the LBA when charging and leave it unplugged when storing the battery. When I use the battery I'll plug in the LBA to monitor the voltage and let me know when to swap to another battery and put the drained one on charge.

I think I can cut an access port on the plastic casing of the battery in a place where it wont cause a problem - the hole will be up on the side facing into the drill and wont be seen while the battery is connected to the drill. Currently there are five screws that hold the plastic casing together - 4 around the edge and one in the middle. That middle screw will have to go anyway because the plastic stem that the screw goes into will be in the way of the new battery. I'll cut that stem off and then cut a access hole - hopefully I'll be able to plug the LBA into the balance lead and then tuck it into the battery casing.

I agree that the proposed pack should work very well. I have only two concerns. First, how will you protect against over discharge? (edit: I now see that you are considering a low battery alarm) Second, many (most?) of the Chinese high power lipo cells available to the RC market have very serious quality issues, and I would be very wary of packing a large number of these cells into a small space, then discharging them at high rates close to my body.

From a purely technical standpoint, you should be able to build a pack that kicks serious butt compared to the original NiCd pack. However, I think that you should be aware/concerned about the potential safety implications of using these low cost lipo cells in the manner in which you are proposing.

Cheers,
BG

I agree, the safety aspect should not be overlooked here. Maybe I should edit the OP to add a warning for anyone thinking of copying my idea.

Well, my LiPo batteries arrived today and I've got one of my cordless drill batteries up & running. It seems to work quite well, but I'll have to test with a decent load (drilling into something fairly hard or using the saw to cut some wood) and see how it does.

So far the one issue that I have found is that the LBA goes off when the battery is under a load even freshly charged - clearly the LBA is set for an appropriate 'no load' voltage which is different to an acceptable 'under load' voltage. I'll have to experiment and keep a close eye on battery levels, maybe I'll just have to swap the batteries relatively early to avoid draining them too low.

I see you have 12 cells so must be a 14.4v system. How does the voltage under load compare between the two kinds of packs now ? NiCD vs LiPo

I might try with my old drill but it is only 13.2v system. My motor might die an early death due to too much voltage.

i doubt it'll be an issue, fully charged nicd 14,4v pack is about 16v as well, lipo 16,8. motors are not that sensetive to 1 extra volt.