Charging 2 Dewalt batteries in Series..
This doesn't seem like it would work, but I've been surprised enough times by these batteries to pose the question anyway:
Can I wire 2 36v Dewalts in series for 72V and still charge both of them with 1 stock charger while they're wired like that? In other words, will the stock Dewalt charger handle a big 72V battery when it's used to 1 36v battery?
My current setup for an ebike battery pack is 2 36v Dewalt batteries wired in parallel (bypassing the BMS and discharging directly from the cells). When I charge these I simply set either one of them on the charger and leave them connected in parallel. They are in fact hardwired together. This charges both batteries (in 2 hours instead of 1) but bypasses one of the BMS's. As long as I consistently switch which battery I set in the charger, can anyone guess as to whether or not that's going to hurt my battery life? My theory is that the amount of times I cell balance is similar to people using these in the field - construction or whatever - they are not leaving the battery on the charger long enough to benefit from cell balancing every time they charge it, and I'm swapping the battery that receives the benefit.
I'm contemplating the same questions. Just got my bike together last weekend. Currently have a 408 motor with one Dewalt battery -- another on the way.
One reasonable solution would be to add a Charge/Run switch to the system. In Charge mode the batteries are configured in parallel and disconnected from the controller. In Run mode they're connected in series to the controller and disconnected from the charger.
I like your idea of charging two batteries with one charger. Simple and effective. Makes sense. And from various reports of RC-enthusiasts and others, seems completely reasonable. Some RC folks with lots of instrumentation find that balancing their packs only occasionally is sufficient. (I found the idea of paralleling the balancing connectors of all the packs overly complex and somewhat questionable, especially for a series configuration.)
BTW: I got a Drainbrain (name recently changed to CycleAnalyst) along with my motor/controller from ebikes.ca -- it's a wonderful tool for trying new configurations. Allows important parameters to be changed at any time, cutoff voltage, max amps, max speed. Very cool.
Russ' video had a lot to do with my selection of parts. The very first ebike I've seen in action was the one I just put together. Not many around here, yet.
I suspect I'll end up with 4 batteries (2s 2p) also. My controller is only rated at 20amps though. I hope that's enough for the performance I want.
Your 2 batteries should be very similar to his 4, except for range. The Dewalts seem very capable of providing plenty of power as long as you bypass the internal 15amp fuse.
(So if everything stops working suddenly you've probably popped the battery's internal fuse.)
Be glad to show you my contraption when you come ...
I'm new to this forum. I have an electric recumbent, and I just hooked up 4 dewalt 36 volts batteries in parallel. I took it for a ten mile bike ride today. It used about 250 watts, 7 amp hours. At peak acceleration it drew about 1100 watts, 36 amps, 30 volts. I ran each of these through a 10 amp fuse and nothing blew. (I had blown a pack previously when only 2 batteries were connected.) I'm impressed with their performance.
Now, I would like to charge them all together, instead of individually. I'm hoping someone can help.
BTW, the tools work great too!
First, some things from your previous post ... Yes you need the Crystalyte controller. Your brushless hub motor has no brushes -- no commutator to control the motor. With a brushless motor the controller is actually half the motor. It handles all the timing that's normally handled by the commutator.
I saw that stuff about the Dewalts having a built in controller -- interesting stuff. But that would most likely only be useful for simple brushed motors.
The good news is my second battery arrived in the mail yesterday! Went for a ride and it was amazing. Climbs hills pretty well. I really gave it a workout.
Bad news is when I got back the charger rejected both of my batteries as bad. :-(
I've read elsewhere that the Dewalt charger rejects batteries that are imbalanced. So I probably need to disassemble them and manually charge the low cell to a level the charger can handle. Need to figure out how to keep this from happening in the future. Not sure if a diode between the two batteries would help or not. ebikes.ca sells them -- guess it would be good form to use one.
Need a better charger/balancer. These batteries are capable of more than Dewalt's circuitry allows.
There is a heat sensor in the dewalt packs. You might try charging them when they cool down. Just a thought.
Still says it's a bad battery. That one is showing 28.5 volts now, was 27.1 after the ride yesterday. I opened the other one up and it looks like one cell is a little lower than the others. Well within limits for A123 but not for the Dewalt charger.
Saw a posting from someone that had a similar problem. He charged the one low cell to get it closer to the others and then the Dewalt charger took it fine.
The tail end of my ride yesterday was up my long steap hill. So the batteries were at their greatest loading just before they hit cutoff -- which I'd set to 54 volts. They were warm at the end, and the Crystalyte controller (48v/20amp) was hot -- need to take it out of the bag and bolt it to the rack for better cooling.
Dewalt's 36-volt drill cuts off at 27volts, so doubling that to 54 seemed more than reasonable for two batteries. A123 Systems suggests 20volts (2v per cell), so 27 is quite conservative. I had my CycleAnalyst set to limit current to 20amps. Its log shows max amps actually hit 23.75 at some point.
Anyway, I need to figure out how to slide the cells out of their press fit in the bottom of the case or get a pinout for the internal balancing connectors so I can charge up the lagging cell.
I guess pulling 20+ amps from a Dewalt battery is more than their scheme can handle.
The good news is I got the first battery apart and charged the low cell to 2.6volts with a couple of AA batteries in series and now the Dewalt charger likes it again.
Voltages of the other cells were all over the place:
2.938 2.862 2.758 2.831 2.875
0.499 2.904 2.872 2.837 2.917
As others have said, looks like the Dewalt charger can't be trusted to do a good job of balancing ...
Maybe that's why their cutoff voltage is so high (27volts vs. the manufacturers suggestion of 20volts) and their fuse is so low (15amps.)
I had a similar problem when I was testing a DeWalt pack on a battery analyzer and ran it voltage down to 22 volts. I am pretty sure that the DeWalt battery charger will reject any battery with a terminal voltage much lower than 27 volts. I recharged the pack with another 36 volt pack in series with a 50w brake light.
The charger worked again as soon as the terminal voltage was above 27 volts. I don't believe that the charger rejected the low voltage battery due to cell unbalance since this pack was reasonably balanced.
I run a 2s4p 72 volt bike with a Crystalyte 409 motor and a CycleAnalyst controller. You might want to set your cutoff voltage to 56 volts. I have never had a problem with running the batteries too low with that setting. I suspect that the CylcleAnalyst may be slow in cutting off the battery. They have the ability to ramp their responses rather than quickly cutting off.
This is a great combination of components for a practical bicycle. Now it we could get our hands on the new A123 batteries and build a compact twenty cell 72 volt pack with an integrated charger.....
Here is an interesting design for a 72 volt pack. He has reverse engineered a drill and used connectors from the dewalt flashlights.
I think he is still charging each battery individually.
Thanks for the info, Dugly,
Perhaps it's time to see if the Dewalt charger's voltage threshold can be lowered a bit. Having a 2.7 volt (per cell) cutoff, when the cell manufacturer suggests 2.0 seems a little odd/silly/dumb. Nowhere have I seen mention that a higher cutoff will serve any useful purpose. Judging from some other battery formulations it could even be detrimental.
From my point of view, it's at least inconvenient. Besides having to precharge before using the Dewalt charger, should the voltage fall too low, I like the quick fall-off at the end of cycle on these batteries. The 2.7 volt cutoff makes it less decisive and seems to leave some energy unused.
The cell manufacturer suggests the cutoff can be set all the way down to zero (0) in colder weather.
Thanks for posting the schematic, Matt. I'm in the process of designing my own battery pack connectors as Russ has done. Seems like a job for some kind of pour molded plastic. With my skills more `semi-old world' construction, like wood and epoxy, may be the choice. Still waffling over spending money on diodes to protect the batteries from each other.
If anyone comes across a schematic for a simple charger that would be nice to see. A small, fast, custom charger to handle four or six packs simultaneously would be nice.
Information on accessing balancing information from the external connector would be really cool too.
I would like to build a 70v pack from scratch, but have decided to wait until I can get the larger 11+ a/hr batteries and only use 20 of them. Meanwhile, it would still be handy to charge all of the battery packs at once.
Does anyone understand the role of the DeWalt battery charger with respect to the battery management system? The batteries want to see constant current during the initial charge and then constant voltage at the end of the cycle. Does the BMS do all of the work through its multiplexor or does it simply provide balancing at the end of the initial charge cycle? Is it possible that the charger is only providing a constant current source and the rest of the control is within the BMS?
If it is the case that we only need to provide a constant current source to the BMS circuits, we might be able to boost the current source and charge all of the batteries in a shorter period of time while retaining the balanancing function to prolong their life and capacity.
If you are using 4 packs for 70 volts, a 400 series Crystalyte motor, a 20 amp controller, and drawing your power though the battery packs battery management controller you will not exceed 15 amps through the fused circuit of the BMS. Therefore, it should be possible to use unmodified battery packs as described in Radtke paper, but modify his circuit to switch the packs for charging.
To me, the desirability of this approach would be to have a sealed, compact battery box that does not have to be dis-assembled for every charge. If the design was done right, it would provide a housing for unmodified battery packs and a circuit board for charging and switching and the packs.
Has anyone reverse engineered the DeWalt 36 volt battery charger? I know that some infomation on the BMS is available.
> I would like to build a 70v pack from scratch, but have decided to
> wait until I can get the larger 11+ a/hr batteries and only use 20 of
> them. Meanwhile, it would still be handy to charge all of the battery
> packs at once.
The good thing about the Dewalt packs is they are available now and
relatively cheap. Larger cells would be great, but no telling when
they'll be available and for how much.
> Does anyone understand the role of the DeWalt battery charger with respect
> to the battery management system? The batteries want to see constant
> current during the initial charge and then constant voltage at the end
> of the cycle. Does the BMS do all of the work through its multiplexor
> or does it simply provide balancing at the end of the initial charge
> cycle? Is it possible that the charger is only providing a constant
> current source and the rest of the control is within the BMS?
> If it is the case that we only need to provide a constant current source
> to the BMS circuits, we might be able to boost the current source and
> charge all of the batteries in a shorter period of time while retaining
> the balancing function to prolong their life and capacity.
Several folks have charged multiple Dewalt packs in series from a
single charger. One parallels the balancing connectors, another
switches the `lead' battery from charge to charge so all packs get
balanced periodically. Both approaches seem plausible.
> If you are using 4 packs for 70 volts, a 400 series Crystalyte motor,
> a 20 amp controller, and drawing your power though the battery packs
> battery management controller you will not exceed 15 amps through
> the fused circuit of the BMS. Therefore, it should be possible to use
> unmodified battery packs as described in Radtke paper, but modify his
> circuit to switch the packs for charging.
According to my CycleAnalyst display I've been drawing 20+ amps through the Dewalt packs standard outputs for extended periods with no problems
so far. When/if one of the internal fuses does blow I intend to make
the very minor mod to the pack to bypass it. The A123 cells are rated
at 70amps continuous discharge. I think something like a 30amp fuse is
more appropriate for this application.
When I started testing my setup I had the Crystalyte 20amp controller
inside a trunk bag until I noticed the controller was too hot to touch
after a lengthy hill-climb. After mounting it out in the airflow it
has never gotten more than a little warm. I suspect the 20amp rating
is fairly conservative.
> To me, the desirability of this approach would be to have a sealed,
> compact battery box that does not have to be dis-assembled for every
> charge. If the design was done right, it would provide a housing for
> unmodified battery packs and a circuit board for charging and switching
> and the packs.
I agree. A more finished, more convenient solution is what I'm
targetting too. Considering the cost of batteries for additional range
versus a smart charger I'm beginning to think a better charger merits
A simple, self-contained charger than will handle the whole pack at one
time would be very nice. One that could do the job in 15 minutes would
be even better! With that kind of speed you could worry less about
range as long as an electrical outlet can be found along the way.
These are great batteries. No sense ignoring their capabilities.
> Has anyone reverse engineered the DeWalt 36 volt battery charger? I know
> that some infomation on the BMS is available.