Toy battery improvement

Moose486

Joined Mar 3, 2021
3
Hello,

My son has a hot wheels sky crash set and it uses 4xD size batteries. It works well for 10 minutes, then the batteries are drained. You can let it rest for a while, but the play time after that is terrible and not reliable.

The cost of D size batteries is outrageous. Especially if you have to replace them often.

I thought to myself I'll put that grade 12 physics to use and come up with something better.

My goal is to get my 18v 5 amp hour (20V fully charged) drill battery to power the toy. You can drill all the palings of a 20m fence on one charge and have the luxury to recharge it.

I knew straight away that the battery would burn out the toy and that I needed to come up with a solution.

I am new to actually making a circuit, rather than just using ohm's law in theory. I can take a burn to, so please be as honest as you need to be.

I used a multimeter to test the total resistance of the toy circuit. My result was 3.4 ohms. The toy ran on 6 volts, so I calculated the current the toy would pull as 1.765 amps.

I reversed the calculation using 18 volts. To keep the current the same, the total resistance would need to become 10.2 ohms. So somewhere before the actual toy, I need to make a resistance of 6.8 ohm to soak up 12 volts before it hits the toy.

I brought a bag of random resistors and they are all rated to .25W. But when I started some more math I hit a problem.

The total power with the 18v battey is 31.77W. The 12V section needs to soak up 21.18W while the 6V section uses 10.59W. That's alot of resistors.

I have smashed google for a solution but I'm not getting anywhere.

I just don't know if I'm doing this the right way. 10.59W for a toy seems alot. Any thoughts of a potential solution to this will be very much appreciated.

Cheers.

BobTPH

Joined Jun 5, 2013
7,557
Let’s see. D cells are about 1.2Ah at 6V or 7.2 Wh.

The drill battery is 5Ah at 18V or 80 Wh, so about 10 times the capacity.

So, you are on the right track. As the previous poster said, you would be wasting 2/3 of the capacity is you used avresistor to drop the voltage. But, that is not the entire problem. The current drawn by the toy is not likely constant, unless it is a very boring toy. So, using a resistor would vary the voltage as the current draw changed.

What you need is a DC to DC converter. This will drop the voltage with only about a 10 to 20% loss of power.

Bob

Alec_t

Joined Sep 17, 2013
13,793
....and there are heaps of DC-DC converters cheaply available from online sources. Seems like you need one with at least a 2A rating. More is better (as it's always a good idea to allow plenty of margin and the toy will only draw what current it needs).

wayneh

Joined Sep 9, 2010
17,406
Another option is rechargeable AAs. Many D cells don't have any more juice than a good AA, they're just physically larger. Get 8 and charge one set while the other set is in the toy.

Otherwise I agree that you should save your resistors for another day. Get a cheap DC-DC converter that meets your needs. Amazon has tons of them.

BobTPH

Joined Jun 5, 2013
7,557
Another option is rechargeable AAs. Many D cells don't have any more juice than a good AA,
That would be if the D cells are the old Zinc carbon cells. A name brand alkaline D cell has about 5 times the capacity of an AA of the same type.

Bob

wayneh

Joined Sep 9, 2010
17,406
That would be if the D cells are the old Zinc carbon cells. A name brand alkaline D cell has about 5 times the capacity of an AA of the same type.

Bob
Ok, maybe so. For rechargeables, most Ds are just AAs in a bigger can.

Moose486

Joined Mar 3, 2021
3
Thanks to everyone for your help. I really appreciate all the ideas. The DC DC converter is new to me so I will give that a go. When I get that working I'll post my results. Thanks again

shortbus

Joined Sep 30, 2009
9,947
Why not just use a power supply? I know the 4 D cells are ~6V but the toy would probably be happy with a 5V DC power supply.