Rebuild 36V 2.4 AH NiCd to new technology

[alternety]

I have a power tool that has a 36V 2.4 AH NiCd battery pack. It takes 24 hrs to charge and runs the tool for 15 minutes. The rough size of the batteries themselves is about 6" X 2.5" X 4". It is made of 3 stacks of 9 round cells. The battery gets quite warm on both charge and discharge. They also put a thin layer of foam around the batteries. Probably to keep rattling to a minimum. But I really question the impact on heat dissipation.

I would like to build a battery pack inside the existing battery case, with at least 3 times the energy content at those discharge rates. NiMh would be easiest, but not a big capacity multiplier. It would be interesting, although not cheap, to do something with a lithium chemistry. I am not a battery expert; and would appreciate some thoughts on feasibility and possible cells/batteries. I understand that this would require a different charger. The tool's charger says it will do NiMh.

 

[Robin24k]

For one thing, it will take less cells to get lithiums up to 36V, and capacity can be resolved with a parallel circuit. However, the problem is charging, and you'll need protection for the pack. There's a reason NiCd and NiMH are still popular among power tools...lithium-based batteries require a completely different system and set of voltages. They are slowly gaining popularity, but your best bet would probably to either find suitable NiMH replacements, or buy another battery pack.

Personally, I would get another battery pack. While rebuilding the pack may be a challenge or project of sorts, the saved money isn't really worth the safety issues and hassle. Be sure to check Ebay, prices there just might tip you towards getting a new battery.

 

[ronkar]

This place makes custom packs with sub-c cells, 36V 4200mAh NIMH.
Would something like this fit inside your box?

http://www.all-battery.com/36v4200mahnimhbatterypack-squrecustomize11829.aspx

 

[Battery Guy]

Your problem with wanting to go to lithiums is in the charger and pack electronics. NiCd and NiMH cells are self-balancing. This means that every time the pack is charged, all of the cells end up at 100% state of charge. The problem with lithium-ion is that the cells do not self-balance. Over time, if you are charging a series string of cells, one or more cells will become overcharged, leading to significant safety problems. This means that you need to have pack electronics that monitor the voltage of each individual cell.

Forget rebuilding the pack with lithium-ions unless you are interested in a major project. You should be able to find high quality NiCd cells to replace your old ones, then you will be back in business. Otherwise, you might check to see if your power tool manufacturer sells lithium-ion packs and chargers for your model.

Cheers,
Battery Guy

 

[alternety]

The existing pack is not going bad - it is new. I am not trying to repair it. Another battery (at >= $100) with the one I already have, will get me a total 30 minutes; not enough. This is the design performance and no, the manufacturer does not have a useful replacement. The design IS the problem.

Going to 4.2 AH NiMH would get me less than 30 minutes and possibly faster charging.

With Lithium I would use a new external charger. One that is capable of dealing with multiple cell charge balancing. I can add new charging points to the physical pack.

I would appreciate some help with ideas on that solution. If it turns out to be too hard - I won't do that. But that is the solution I am trying to get some assistance with. From what I recollect from various researches into the technology, I do understand the dangers of mishandling them. I believe there are also some chemistries that are safer than others.

I appreciate the warnings, but if some of you guys could please talk to me about dealing with the issues rather than "don't do that" I would really appreciate it. It is really the only solution worth pursuing. I don't play with batteries often, but I am an old electrical engineer. I do know how to deal with things.

It is also possible to just build an external battery pack. I could live with that. It certainly would vastly simplify construction not having to fit things into the existing case. A direct substitution of sub-C cells in NiMH would be easy, but not enough power. I would have to leave an unused battery pack in the tool (a string trimmer) to maintain tool balance. Again Lithium would be the preferred solution because of weight and the other features. The existing pack is quite heavy.

 

[Glasstream15]

Buy a gas or 110 volt trimmer. Not a joke. I tried the battery rigs. Even the Ryobi lithium is not really satisfactory. Gas works really well. No extension cords. And you can get a good gas trimmer for way under $100.

 

[Mattmanic]

I use lithium ion batteries and my $10 charger will do 7.4v and 11.1v batteries and ballance them. The cost of using lipos has really gone down recently. That charger, or 3, and 6 good 11.1v batteries will be able to give you 33.3v @ 5AH without costing a massive amount, although the batteries will be a lot, and they'll be able to discharge at 50-100A.

 

[alternety]

OK; price check. I have spent the afternoon looking at pricing. Li based batteries appear to be just too expensive to use. I looked at NiMH and it appears the sweet spot is about 10 AH D cells. F cells seem to have a marginal price/AH that is worse that the Ds.

Can anyone help on brand and source? I have searched this forum and not found a whole lot that is definitive. I have seen Tenergy mentioned fairly often. They seem to be available at reasonable prices. Low discharge is not really necessary for my use. I have also seen Powerex. Any links to tests or good reviews?

 

[HarryN]

SAFT and Sanyo have some pretty good NiMH cells in those larger sizes.

There are some cells designed for "max discharge" and others for "capacity". I have 2 packs from cheapbatterypacks.com that are 5 AH sub c cells. I have to admit not really testing them fully but they work well for family r/c use.

That fact that you are now getting 15 minutes from a charge is a good indication that they are well under capacity in general. My guess is that the rated 2 AH you are seeing now is optimistic compared to reality. A better cell will likely help a lot.

I suppose you could also attempt to run from a "local stationary pack". 3 decent car batteries in series can hold a ton of charge and hit your 36 volt goal. 50 AH is pretty common in a good car or marine battery. The portability can be improved by mounting them on a cart and wire them over to the power tool. A relatively common car charger can easily pump 50 amps for fast charge ups.

 

[alternety]

The 2.4 AH rating is probably in the ball park. The motor draws enough current to drain them in 15 minutes. That is probably a good ways down on the graph of current vs recoverable capacity. And of course, I do not know what current is actually being drawn. I would need to disassemble the trimmer to measure it. I would prefer not to remove the anti-tamper stickers. But just based on run time I am guessing in the range of 10 A.

As an aside, carrying 3 car batteries over my shoulder is not something I really want to do. If I had flat terrain, I would use a mower.

 

[alpg88]

i use tenergy d, and c cells in my lights, i can't complain, they work just fine, however i don't draw 10 amps, but they are good for 30 amps on paper, so imo 10 amps will be fine.

you will need 27 cells, about 10lb, looks like you will have to carry them on your back,

 

[ifor powell]

The best ready made battery I see matching you is this http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=11037 a 10S 4500 mAh LiPo Not at all bad value at $99. size want fit in your box. The problem is you will need a hobby charger with 10S. Two 5S packs may be better e.g. somthing like http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=9174

 

[alternety]

ifor, those look interesting. I would lean toward the 2 X 5s for brand and price. From what I have seen the smaller back brand is better thought of than the other.

But I am not sure I could charge them together in series and have all the balancing work. I am leaning toward the iCharger 1010B. I am open to other suggestions (like a quality clone). I don't see anything in the manual for the charger about charging 2 X 5s in series. My gut feeling is it should work. In my mind, the connection between the one battery and the next should be invisible to the charger. Anyone know if this is true or if I am missing something?

I am going to try to insert some wires into the trimmer and monitor the voltage on the battery under operating conditions. I am concerned about two things; maybe 3.

The batteries are LiPo. Full charge voltage is 4.2 V. I am concerned that a fully charged pack may exceed the voltage capability of the trimmer controller. Using 9 cells gets the voltage too low.

I need to measure the actual cutoff voltage for the trimmer controller and make sure it stops the motor before it damages the LiPo cells. I think I am OK there.

Then there is the question of how much usable run time I will get. That is going to depend, at least in part, on what method the controller uses to run the motor.

 

[HarryN]

LiPo based packs can be great, but I wouldn't touch one without doing it "correctly", as in balancing taps and a very good quality charger. (at least a Triton II or better)

It is nearly certain that a LiPo or Li Ion pack with have a "bad event" without the specific safety precautions.

Some of the Li based cells (IMR, etc) are more tolerant of this, but I still would be hesitant to get too far away from balanced charging.

I know this isn't what you want, but perhaps running a long cord would be cheaper, safer and faster. 10 amps at 36 volts is easy with a wire.

 

[Stereodude]

Buy a gas or 110 volt trimmer. Not a joke. I tried the battery rigs. Even the Ryobi lithium is not really satisfactory. Gas works really well. No extension cords. And you can get a good gas trimmer for way under $100.

FWIW, I've got an 18V B&D cordless string trimmer and I only have to charge the battery a few times all spring, summer, & fall.

 

[ifor powell]

But I am not sure I could charge them together in series and have all the balancing work. I am leaning toward the iCharger 1010B. I am open to other suggestions (like a quality clone). I don't see anything in the manual for the charger about charging 2 X 5s in series. My gut feeling is it should work. In my mind, the connection between the one battery and the next should be invisible to the charger. Anyone know if this is true or if I am missing something?

Chargeing the packs joined up together is OK. I do this with 2 3S packs. The balance board I have has 3 sets of 3S lines up and 2 lots of 4S and 5S. I have an iChartger 208B and am very happy with it. It's nice and robust and has suvived me doing some stupid things with it like pluging in the balancing pait of ballancing leadt the wrong way.

I am going to try to insert some wires into the trimmer and monitor the voltage on the battery under operating conditions. I am concerned about two things; maybe 3.

The batteries are LiPo. Full charge voltage is 4.2 V. I am concerned that a fully charged pack may exceed the voltage capability of the trimmer controller. Using 9 cells gets the voltage too low.

I need to measure the actual cutoff voltage for the trimmer controller and make sure it stops the motor before it damages the LiPo cells. I think I am OK there.

Then there is the question of how much usable run time I will get. That is going to depend, at least in part, on what method the controller uses to run the motor.

The max voltage should be good. Theoreticaly your curent pack off the charger may be 42V (1.4v per cell) But best to test what it is in practice.

Yes low voltage may be an issue you need to check this.

 

[was.lost.but.now.found]

The max voltage should be good. Theoreticaly your curent pack off the charger may be 42V (1.4v per cell) But best to test what it is in practice.

Hang on there. I was about the agree with you because normally the spec voltage is simply a matter of 1.2V * #batteries. So in an 18v pack you'd normally find 1.2V * 15 = 18.0. Hence you would expect 1.2 * 30 = 36.0 (and hot off the charger 1.4 * 30 = 42.0) but the OP said 3 stacks of 9 cells each which is only 27 cells. That's 1.2 * 27 = 32.4 nominal, and 37.8 hot off the charger. That 42.0V may indeed be a little 'hot'.

 

[alternety]

The 18V trimmers are pretty much a completely different beast. All discussion is relevant only to the 36V. Wandering randomly around a couple of acres of plants, bushes, rocks, walls, ponds, hills, gullies, buildings, cars, wood piles steep banks, ponds, tanks for propane, diesel,water, septic controllers, more plants, trees, roses, critter enclosures, stairs, etc., does not make a particularly friendly scenario for a 400' extension cord. Please address the stated topic. I have a very good gas trimmer - I WILL NOT USE IT. That is not open to discussion. I have a battery trimmer - I WILL USE IT. Ditto.

For those actually addressing the questions; thank you very much.

About the 1.2V cells. When I had the B&D pack apart I believe I counted 27 cells. That does not make a 36V battery. Is is 32.4. That leads to some annoying questions of what number of what cells to use. Or I need to take the pack apart again and make sure I counted correctly. As we speak, I have a battery fresh off the charger that shows 41.7V. That smells like 30 cells.

Someone has pointed me to some supposed A123 cells at a reasonable price ($120/20). They appear to have been removed from a battery pack. Presumably a Dewalt or B&D pack. One has to wonder why they were removed; and if they are really A123, and what condition they are in. If I paralled 2 packs (10S2P in the battery vernacular) I would get about 5AH. That would only get me about 1/2 hr in my mind. But they could be recharged in 15 minutes to an hour depending on temperature. At the low end, good for a beer break. At the high end, a beer break could lead to a full day recharge. The A123s really do seem to be a revolutionary technology.

I also have to do some more research on what actually happens in the cell balancing circuits. This whole thing got way more complicated than I had hoped. I have a very large area of rye that is 6' to 8' tall (e.g. my whold cleared lot). I had no idea it could get so tall. I planted it last fall with vetch as a winter cover. By the time it rained enough it was too cold. No winter cover. We had a cool and abnormally wet spring/summer. I have not been able to cut it down and it is seeding. 15 minutes a day are not getting it done.

 

[alternety]

The company selling the A123 type batteries is Techex on ebay. They are a paper case looking a bit primitive, but I seem to remember A123 used to or does look like that.

Has anyone had any experience with these batteries?

 

[alpg88]

idk if you wanna buy batteries removed from a pack for unknown reason, you have no idea how and how long they were used.

40+v off charger does sound like 30 cells.