Drill battery build help - Noobie seeking direction

Batteries, Chargers, and Battery Management Systems.
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lameduck   10 mW

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Drill battery build help - Noobie seeking direction

Post by lameduck » Mar 12 2018 9:52pm

I am starting to learn more about building batteries. For my first build, I thought I would swap out some old NiCad batteries for Lithium cells. I have a BMS for a 4S battery. I am looking for direction on how to attach the P+ and P- from diagram attached here, the P+ hookup seems pretty obvious, but in looking at the existing NiCad (picture attached) the negative wire is attached to the battery. My question is do I attach the P- on the BMS to the connector on the existing NiCad battery that has the negative wire attached to it? Hopeful that all makes sense :-)
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Sunder   100 MW

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Re: Drill battery build help - Noobie seeking direction

Post by Sunder » Mar 12 2018 10:53pm

Either you're over-thinking it, or I'm under thinking it.

P- usually refers to the negative of the load.

Since your "load" doesn't exist until you plug your battery into your drill, you solder a wire between the BMS point marked P- and the negative terminal of the finished pack. (not the raw battery pack).
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Re: Drill battery build help - Noobie seeking direction

Post by amberwolf » Mar 13 2018 1:25am

lameduck wrote:
Mar 12 2018 9:52pm
I am looking for direction on how to attach the P+ and P- from diagram attached here, the P+ hookup seems pretty obvious, but in looking at the existing NiCad (picture attached) the negative wire is attached to the battery. My question is do I attach the P- on the BMS to the connector on the existing NiCad battery that has the negative wire attached to it?
P+ will go to the contact on the drill pack that has the red wire.

P- goes both to the contacts on teh drill pack that has the black wire farthest from teh red one.

Make sure to remove the NiCd cells first. ;)

Note that the negative contact of the drill pack up in the plastic tower is probably spotwelded to the most negative NiCd cell, so you will probably have to cut it away from the cell.

The thermistor is no longer needed, and it's unlikely the drill checks it (just the charger), so you could probably remove it if you like.

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Re: Drill battery build help - Noobie seeking direction

Post by lameduck » Mar 16 2018 10:48pm

Thanks for the replies. That helped me get it wired up. It is now all wired, and I am getting 14.9-15V from the battery when measured...but when I put it in the drill...I get nothing??? I am I missing something? The NiCad battery says that it was a 14.4V battery??
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Re: Drill battery build help - Noobie seeking direction

Post by amberwolf » Mar 17 2018 12:25am

lameduck wrote:
Mar 16 2018 10:48pm
It is now all wired, and I am getting 14.9-15V from the battery when measured...but when I put it in the drill...I get nothing???
By "get nothing" do you mean it measures as 0V at the BMS terminals when it's in the drill?

Or do you mean that the drill does not spin?

Or somethign else?

Even if it's wired backwards it would probably just spin backwards (most of the old nicd ones were just brushed motors and a switch in the trigger; few have any polarity sensitive components)
The NiCad battery says that it was a 14.4V battery??
That's a 12 cell battery. 1.2v nominal on each one is 14.4v, so your pack should be the right voltage.

There is a potential issue with the BMS--a motor has a high surge current when turning on, especially under load. The BMS might trip on overcurrent and shutdown, if it's not rated high enough current.

Does the drill or nicd pack list the rated current it draws?

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Re: Drill battery build help - Noobie seeking direction

Post by lameduck » Mar 17 2018 9:46am

Sorry for the lack of clarification. Yes, by "get nothing" I mean when I put the battery into the drill does not spin and when I put the battery on the charger, there is no recognition that the battery is on the charger (no blinking lights).

I do measure the correct voltage of 14.8-15V from the connectors on the battery though...

The BMS was listed as a 30A BMS.

From what I can gather, the Dewalt DW928 has the following specs:

Voltage = 14.4
Watts = 280

The Amp draw should be ~20AMPs....I am I calculating that correctly?

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Re: Drill battery build help - Noobie seeking direction

Post by amberwolf » Mar 17 2018 1:04pm

Sounds right, for continuous operation. If it were only teh drill that wasnt' working, I'd suspect just a surge current causing the BMS to shutdown.

If you can keep a voltmeter attached to the BMS output wires while you've got it in the drill, and then watch the meter while activating the drill: If the BMS output goes to zero then it's shutting off to protect the pack from something--overcurrent or cell-LVC. If it doesnt', but the drill doesnt' do anything, there's something the drill needs that it isn't getting from the pack.

Regarding the charger, if you're using the original charger, it probably won't operate without the thermistor, because that is a really big safety thing with the NiCd stuff--without it, the charger doesn't have a way to know the NiCds are' getting hot enough to catch fire and stop pushing current thru them. (it's simpler to do just a thermal check than a delta-V check, which makes the chargers cheaper to make)

Regarding the drill, maybe it also has actual electronics in it that are powered by the pack, and read the thermistor for overheating in use, in which case you'd also need to leave it in place.

If you've removed it it's easy to add back in, it's not polarity sensitive. Just wire it between ground and the unused pin on the pack's terminal block.


If it's not that, then I don't know--maybe the pack's original wiring was backwards, so the negative was on red, and positive on black, and there are diodes or other polarized components in the drill.

The old NiXX stuff I have here is much simpler than that, and doesn't care whcih way I hook up a LiPo pack to run it. ;)

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Re: Drill battery build help - Noobie seeking direction

Post by lameduck » Mar 17 2018 4:05pm

Thanks for all the replies and help so far. I hooked up some alligator clips to the drill and the battery. Looks like the wire on the P- discharge port was not thick enough to handle the load.

Now that I got that problem fixed, I had another question/problem to try and figure out.

I noticed that hooked directly up to the P+ and P-, I cannot fully compress the drill and get instantaneous torque and speed right away. The BMS cuts power if I try to draw that much power quick. I can gradually bring the drill up to maximum speed though.

If I hook the drill up directly to P+ and the B-, then I can pull full power right away.

Is there anyway around this limitation from the BMS? Can I bridge the B- and P- pads/wires so that I can draw full power instantly?

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Re: Drill battery build help - Noobie seeking direction

Post by Sunder » Mar 17 2018 4:40pm

Tempted to say bypass the BMS if you know the cells are good for it, which is fine 99% of the time. But the manufacturers rely on the BMS to protect not only the battery, but by default the motors as well. They know the BMS will cut power before the motor burns out. If you go way oversize or no protection, you can burn out a tool.

Really, the best option is to get a BMS not more than 10% bigger than the original, but that could be hard to find one that fits. Or, you could go unprotected and only use the BMS for balancing, but that risks both tool and battery.
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Re: Drill battery build help - Noobie seeking direction

Post by amberwolf » Mar 18 2018 12:56am

Keep in mind that his original battery had no BMS, being NiCd. ;) So this BMS wasnt' meant for a power tool at all.



@lameduck:

It's just as likely that it's the cells themselves that cant' handle the load of hte drill--they may be sagging in voltage so much that they hit LVC. You can check this with a voltmeter on each cell (or each balance wire pair on the BMS), while applying the load. If the voltage drops very much, then they're just not high enough C-rate cells, and you need more in parallel.

As for bypassing this BMS, you could do that, by wiring the main battery negative to the drill negative, instead of wiring the BMS negative to the drill negative. (I wouldn't short across any of the BMS pads, but rather just run the wires to where you want them).

But then the BMS can't protect against a cell going too low.

It also can't protect against a cell going too high, if you are using the drill charger thru the same terminals the drill is powered from.

As long as the pack is well-balanced and you manually check voltages on the pack or cells during use and charge to be sure it doesn't get overdischarged or overcharged, that's fine--but if the cells are being used near or at (or beyond) their maximum specs, they're likely to wear quicker and become unbalanced quicker, with more risks of using them BMSless.


Note that if you leave the BMS on there with teh cell sense wires connected, then it will still be able to balance the cells that go high enough to be slightly overcharged. But it can't shut off charging or discharging, if it's bypassed, should something go wrong at cell-level, like it normally would.

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Re: Drill battery build help - Noobie seeking direction

Post by Julez » Mar 23 2018 9:58am

Hi! I finished a very similar project a while ago:

https://imgur.com/a/GBcvW

Image

Image

I also had the problem that the BMS cut out prematurely under load. A test revealed >100A are possible at full throttle. So I bypassed the BMS for discharge. For charge, a 3rd contact at the top of the battery pack was installed, the charger already had a counterpart at that position. So far, things are working excellent.

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Re: Drill battery build help - Noobie seeking direction

Post by fechter » Mar 23 2018 2:53pm

lameduck wrote:
Mar 17 2018 4:05pm
Thanks for all the replies and help so far. I hooked up some alligator clips to the drill and the battery. Looks like the wire on the P- discharge port was not thick enough to handle the load.

Now that I got that problem fixed, I had another question/problem to try and figure out.

I noticed that hooked directly up to the P+ and P-, I cannot fully compress the drill and get instantaneous torque and speed right away. The BMS cuts power if I try to draw that much power quick. I can gradually bring the drill up to maximum speed though.

If I hook the drill up directly to P+ and the B-, then I can pull full power right away.

Is there anyway around this limitation from the BMS? Can I bridge the B- and P- pads/wires so that I can draw full power instantly?
The BMS has a current measuring shunt somewhere on the board. Add some additional shunt resistors in parallel to increase the trip point. I've done this on several BMS boards that were a little too sensitive. This way you still have protection against over discharge and short circuit.
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