# Charging 120 AA Nimh cells for electric longboard build

#### Renbock

Joined Jun 7, 2018
7
Hi there

Studying Mechanical engineering but with a keen interest in electronics. Want to convert my longboard to an electric longboard with rechargeable AA batteries as They are cheap to buy, and I think it will be fun to do something a bit more interesting with them rather then remotes and torches

So. I have discovered that most chargers can only handle 15 cells maximum, bummer! But what if I make my circuit so it only charges 15 at any given time?

I have included a crude drawing of what I mean. Each rectangle. Aside from the one that is shaded in. Represents a 15 cell block of Nimh batteries hooked up in series. Would it be possible to use a arduino and transistors (or other device). To allow current to flow through one block over a given time period, say 2 hours? Or, even better, is it possible to create a circuit where once a block of 15 batteries are charged, the arduino diverts power to another block of batteries and begins to charge those? until such time as all batteries are charged?

In the drawing you see 2 transistors, but in reality each block of batteries would have its own transistor.

For the circuit during use. I intend to put place 37 Nimh batteries in series for a total of 44.4 volts (Max my motor can handle)

And connect three packs of 37 in parallel for a total of approximately 8-9 amp hours. The total current drawn should not reach any more than 51 (as max power 2300 according to website. 2300/44.4=51) However the ESC I am using can handle way above this (as stalling may be higher). In fact as each cell will likely give off just below 1.2 volts. I am tempted to just use 40 instead.

The charger I will be using is this

Below are questions I have for you guys

1. assuming batteries are functioning normally and fully charged, will this setup provide enough power without anything going bang?

2. Can i use this set up to charge my battery packs, one after another with transistors using and arduino? And is there an easier way to do this?

3. Are there any holes in my thinking? I really have not a scoopy doo what I'm doing, so any input is appreciated

Thank-you so much!

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#### Alec_t

Joined Sep 17, 2013
14,009
Welcome to AAC!
The motor in the link has a rated maximum continuous current of 40A. Its start-up current will likely be several times that.
Regardless, if we assume the motor will be drawing, say, 30A then each cell of each of your three battery strings, if balanced, would have to provide 10A. I don't think your little AA cells could do that without overheating and rapidly dying .

#### Hymie

Joined Mar 30, 2018
1,229
Welcome to AAC!
The motor in the link has a rated maximum continuous current of 40A. Its start-up current will likely be several times that.
Regardless, if we assume the motor will be drawing, say, 30A then each cell of each of your three battery strings, if balanced, would have to provide 10A. I don't think your little AA cells could do that without overheating and rapidly dying .
That’s the smallest 2.3kW motor I’ve ever seen – just how fast does this skateboard go?

#### Renbock

Joined Jun 7, 2018
7
Welcome to AAC!
The motor in the link has a rated maximum continuous current of 40A. Its start-up current will likely be several times that.
Regardless, if we assume the motor will be drawing, say, 30A then each cell of each of your three battery strings, if balanced, would have to provide 10A. I don't think your little AA cells could do that without overheating and rapidly dying .

Ah, so what your saying is I need MORE POWER!!! Perhaps if I had four packs five packs that might do the trick. I've been trying to find the maximum safe discharge rate of Nimh AA batteries all across the web, but am yet to find a good resource, perhaps you might know where to start looking for this kind of thing? But yes, you are likely correct

The other option is to split them into 6 packs each supplying approximately 22 volts. Then each string would supply 5A. However, in this configuration. Is it correct to say that the maximum power output would be in the region of 22 volts * 30 A = 660w. Or 22*40=880?. As the maximum power rating for the motor is 2300w would I be right in saying that I should attempt to supply the maximum voltage possible, whilst also allowing an adequate current to flow? If I use 22 volts how much power might I lose?

#### Renbock

Joined Jun 7, 2018
7
That’s the smallest 2.3kW motor I’ve ever seen – just how fast does this skateboard go?

Well I'd like it to go 30mph on a flat. As 2300W is approx 3 HP I'm hoping that I can reach that kind of speed. I came to this conclusion as a 125cc motorcycle produces roughly 11hp. Rider and bike comes to roughly 250KG. And thats enough for 60 MPH. So with under half that weight (I weigh 84kg and my longboard roughly 5kg before modding) I should be able to go at least 20 MPH.

That is a good point though. So do you think it is unlikely that this motor is as powerful as claimed? Can I test this when it arrives to inform what battery set up I should use?

#### Hymie

Joined Mar 30, 2018
1,229
You could consider something like this at the heart of your charging system, with it being able to handle 48V charging – you would only need to charge each of your three sets of batteries.

https://www.ebay.co.uk/itm/DC-6-60V...m=291995521114&_trksid=p2047675.c100005.m1851

With a circuit that automatically switches over to the next set, the whole charging cycle could be automated.

Looking at the device, it does not appear to provide any charge current control/limit (only voltage) – this would be provided by the connected mains charger.

#### Renbock

Joined Jun 7, 2018
7
You could consider something like this at the heart of your charging system, with it being able to handle 48V charging – you would only need to charge each of your three sets of batteries.

With a circuit that automatically switches over to the next set, the whole charging cycle could be automated.

Looking at the device, it does not appear to provide any charge current control/limit (only voltage) – this would be provided by the connected mains charger.
So the device you have posted controls voltage as well as current? do I need any other components in order to charge the battery? What should I plug into the battery charger?

Also, do you think the circuit I posted would be a possible way to achieve this kind of automation? Is there a better way? And if so what/where do I need to look?

Thanks again!

#### Hymie

Joined Mar 30, 2018
1,229
So the device you have posted controls voltage as well as current? do I need any other components in order to charge the battery? What should I plug into the battery charger?

Also, do you think the circuit I posted would be a possible way to achieve this kind of automation? Is there a better way? And if so what/where do I need to look?

Thanks again!
Although not wonderfully clear from the listing, what this product does is disconnect the charger once the battery reaches a pre-set voltage.

But the attached power supply/charger would need to be compatible with the batteries being charged. By that I mean that the maximum output current (of the charger) matched the maximum charge current of the batteries being charged.

#### Renbock

Joined Jun 7, 2018
7
Although not wonderfully clear from the listing, what this product does is disconnect the charger once the battery reaches a pre-set voltage.

But the attached power supply/charger would need to be compatible with the batteries being charged. By that I mean that the maximum output current (of the charger) matched the maximum charge current of the batteries being charged.
I see, okay that makes more sense

Would it be possible to use only one of these in the entire circuit? Or would I need one attached to every battery pack?

Also, although I willl use 3 packs of 40 batteries for riding around. When it comes to charging my charger can only handle 15 cells max, therefore I will need a slightly different circuit set up for the purposes of charging, which I have included in the original post

#### Hymie

Joined Mar 30, 2018
1,229
I see, okay that makes more sense

Would it be possible to use only one of these in the entire circuit? Or would I need one attached to every battery pack?

Also, although I willl use 3 packs of 40 batteries for riding around. When it comes to charging my charger can only handle 15 cells max, therefore I will need a slightly different circuit set up for the purposes of charging, which I have included in the original post
The problem with charging batteries in parallel is that one of the parallel paths may take more or less of the one third charging current – resulting in the cells not being charged to their optimum.

That said, it is quite common to have cells in parallel pairs (to increase the capacity of the battery pack) with two charging paths – so I would imagine charging 3 in parallel should be OK.

#### Alec_t

Joined Sep 17, 2013
14,009
...... and the problem with charging cells in series is that the weakest cells gets overcharged and damaged, unless proper charge balancing control is used, which involves closely monitoring the voltages (and probably temperatures) of individual cells.

#### -live wire-

Joined Dec 22, 2017
959
It would really be ideal to operate it at a lower voltage. Here is the problem. Your motor needs 50A and 2kW. I assume the minimum acceptable run time is 20 minutes. Given that capacity is less when discharging at high rates, you need at least 1kWh (2kW for 30 minutes). 1kWh is a lot! I strongly recommend a lower operating voltage or reducing the pulse width (PWM). 50A and 40V means a reactance/resistance total of .8 ohms. power = I^2*r, and V=IR, so you can figure out power and voltage/current from there.

Lead acid could probably give it to you for $100-200. But it would probably be huge and 40 lbs, at least. Lithium, on the other hand, would be 3 times as light and a lot smaller. But it may be up to 4-5 times as expensive. So be willing to pay the price. Thread Starter #### Renbock Joined Jun 7, 2018 7 It would really be ideal to operate it at a lower voltage. Here is the problem. Your motor needs 50A and 2kW. I assume the minimum acceptable run time is 20 minutes. Given that capacity is less when discharging at high rates, you need at least 1kWh (2kW for 30 minutes). 1kWh is a lot! I strongly recommend a lower operating voltage or reducing the pulse width (PWM). 50A and 40V means a reactance/resistance total of .8 ohms. power = I^2*r, and V=IR, so you can figure out power and voltage/current from there. Lead acid could probably give it to you for$100-200. But it would probably be huge and 40 lbs, at least. Lithium, on the other hand, would be 3 times as light and a lot smaller. But it may be up to 4-5 times as expensive. So be willing to pay the price.
Thats some really helpful information. Thankyou.

There are a couple things I am struggling with however.
You recommend a lower operating voltage. I assume that this is because voltage is proportional to resistance? Therefore the lower my voltage the lower the resistance?

Also, If I were to use a lower voltage. Say, 22v, and the motor uses 50A (This is the max continuous current according to there website). Then P=IV, so 22*50=1100. 1100 Watts is well below the maximum power output of this motor. Is there a way to increase this nearer to its maximum of 2KW whilst using a low operating voltage? Also do you know any good resources to better understand this kind of stuff?

one last thing, if using Nimh batteries instead. an alternative would be to use 6 packs of 20 cells (1.2v 3000Mah per cell), each pack pumping out 24v and needing to supply 1/6th of 50A. Or 8.3 Amps, taking into consideration the stall current (150A perhaps?). Is the high current draw why you have suggested lead acid batteries instead? Or is there another reason? Just trying to wrap my head around the whole picture is all.

#### Alec_t

Joined Sep 17, 2013
14,009
According to responses in the Pololu Forum your AA cells will drop a lot of voltage at currents over 1A, hence they will heat up dramatically and be unsuitable for your project .

#### Hymie

Joined Mar 30, 2018
1,229
A world class cyclist can produce an output power of around 400W; and that figure is for an endurance race – with an average speed of over 20mph.

With a power draw of 2300W, I would estimate that with a 75kg adult on the board – speeds well in excess of 70mph would be possible.

#### Renbock

Joined Jun 7, 2018
7
A world class cyclist can produce an output power of around 400W; and that figure is for an endurance race – with an average speed of over 20mph.

With a power draw of 2300W, I would estimate that with a 75kg adult on the board – speeds well in excess of 70mph would be possible.
Hahahaha, Well If I don't report back in a month or two its because I've become mulch on the side of a wall somewhere, otherwise Ill let you know!

#### Hymie

Joined Mar 30, 2018
1,229
Hahahaha, Well If I don't report back in a month or two its because I've become mulch on the side of a wall somewhere, otherwise Ill let you know!
I suspect that the motor power and current figures are out by a factor of 10 – and therefore you might get reasonable mileage out of using your proposed battery type.