Big Motors and Battery Power Supplies - Do I need a voltage regulator?

Sup,

I've got an 18v motor from a Black & Decker leaf blower. Instead of using their overpriced Li-Ion battery packs that only have 4Ah max, I'm going to try to daisy chain a bunch of these batteries instead:

http://www.ebay.com/itm/4PC-UltraFire-5000mAh-3-7v-18650-Rechargeable-Li-ion-Battery-2PC-Smart-Charger-/151293647466?pt=Battery_Chargers&hash=item2339cdda6a

Now, if I hook up 5 of these 3.7v batteries in series, the voltage would be 18.5v. This is pretty close to the 18v the motor is rated at, which seems like it would work ok. The problem is, that's the battery's general voltage. As I learned with a 4000Ah version of these batteries (also rated at 3.7v), the voltage fresh off the charger is 4.2v! It isn't 3.7 as one would expect. That would mean when I first start using the batteries from a fresh charge, the motor will get a whopping 21v instead of the recommended 18v. Is this an issue? What kinds of devices are used to overcome these situations? I've tried looking for voltage regulators, but I can't find one that handles more than 2Amps (I need a 5.15 Amp one). Thanks for your time!

I think you'll find the Black & Decker LiIon batteries charge to the same level - hard to overcome battery chemistry. I think in general all Li batteries charge to 4.1V or 4.2V, and you don't want to discharge below 3.7V. The electronics in a battery pack should prevent overcharging & overdischarging. If you assemble your own, you should include the same kind of charge control. The switch in the handle and the electric motor probably don't care that much.

CrossRoads: I think you'll find the Black & Decker LiIon batteries charge to the same level - hard to overcome battery chemistry. I think in general all Li batteries charge to 4.1V or 4.2V, and you don't want to discharge below 3.7V. The electronics in a battery pack should prevent overcharging & overdischarging. If you assemble your own, you should include the same kind of charge control. The switch in the handle and the electric motor probably don't care that much.

Thanks for your reply!

The batteries specify "Over charge and discharge protection circuit". Assuming these specs are true, does that mean the batteries protect themselves from going below 3.7v by themselves? Do I still need an external circuit to monitor and disconnect them even with their builtin protection? What kinds of hardware are used to build said circuits? Can an Arduino be used to monitor the batteries instead?

EDIT: Apparently the protection circuitry only works if the cells are used alone. Using them together in a battery pack apparently defeats the safety circuitry and an external circuit is required. I'd also have to run a ton of them in parallel since the max discharge for a single cell is 600ma. I found another site that advertises 1500ma constant current, but again I don't know how accurate these specs are. I might just go with some 6v lead acid batteries unless anyone else has any sweet ideas.

Charging Li cells in series requires a charge-balancing circuit, since overcharge of a single cell is enough to ruin it and risk a fire.

You can get units that monitor a Li batt pack and sound a buzzer if any cell is over or under-voltage.

As for the 18V is won't care about being driven from 21V, that's only 14% above nominal - it'll spin 14% faster - the bearings will survive.

Maybe something like this http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9174 Contact them to ask about a charger and overdisharge protection.

MarkT: Charging Li cells in series requires a charge-balancing circuit, since overcharge of a single cell is enough to ruin it and risk a fire.

You can get units that monitor a Li batt pack and sound a buzzer if any cell is over or under-voltage.

As for the 18V is won't care about being driven from 21V, that's only 14% above nominal - it'll spin 14% faster - the bearings will survive.

Awesome, thanks. By the way, is there a general rule to follow for "overclocking" motors? (Like, no more than 20% or something?)

CrossRoads: Maybe something like this http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9174 Contact them to ask about a charger and overdisharge protection.

That looks like a good battery pack. I'm going to search around for general hobby batteries with similar specs and see what I can find.

I have a big blue battery pack (that I've misplaced somewhere) that is 11.1V to 12.6V I think that I use with 3-LED strips, 6.8AH or something big like that. Wouldn't be good for motors tho, not high enough disharge rating like the one above: Constant Discharge: 20C

Keep an eye on that as you're planning to use the battery pack with a motor.

I've used those batteries.

The over-current protection works. I shorted out three stacks of 8 of them, the fuses (on the project) all blew, and then when i checked the batteries, i found them to all be open circuits. After a few minutes, they started turning themselves back on.

AFAICT, the over-discharge does not, nor do I trust the over-charge protection.

That said, if you charge all the batteries up to full charge, and then put them in series and only ever charge them as a group, you can get away with not charge balancing - just give some margin on the low voltage you let it get to (I think I have my project stop at 3.2v under load, and won't turn the load on if it's under 3.7). If these were nice batteries, not pieces of poorly made junk, I'd take more care.

On that note - Ultrafire batteries fall far short of spec. I've got some 4,000 mAh ones that I get like 650 mAh out of, and some 6000mAh ones that give 1500-2000mA. The 6000mAh ones that give 2000mAh are still way cheaper than the alternatives - I'm actually going to order another $100 of them this week. The 4000's that give 650, not so much.

Regarding voltage regulation, depending on how much you care about how much voltage the motor is getting, you might consider using a DC-DC converter to step the voltage up or down to whatever the motor is happiest with - these can be had for astonishingly low prices on ebay. Whether you need that depends on how picky your motor is and whether you're running it continuously.