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This might be a silly question, but here I go. My son’s power wheels car battery (12v 9.5Ah lead acid) is gone and, here in Venezuela, they are pretty difficult to find, not to mention the high prices. So looking for an alternative I found that a local computer store have those laptop battery packs on sale (10.8Vdc 2600mAh/28.08Wh Li-Ion) they also sell external chargers for this battery pack, and I started to wonder if I can build a battery pack that works properly on the power wheels. I been reading about, it takes more than basic knowledge on batteries to do that, so I decided to post this question.

Is it possible to make an arrangement of these batteries to power this type of car?

I know that these type of batteries needs circuitry to prevent overcharge (over 4,2v), prevent over discharge (below 2,5v), charge balancing, and prevent over current (more amps than they support during discharge). What my limited knowledge (mixed craziness) tell me to do is, buy 4 battery packs, 4 chargers (that will solve over charge and charge balancing problems), and use them in parallel to power the car (10.8Vdc 10400mAh 112.32Wh), I suppose I’ll need some circuitry to prevent over discharge, and I don’t know if I need something to limit current since 10.4Ah are over original battery 9.5Ah.

And yes, crazy as it sounds, I can buy 4 laptop batteries plus chargers by half the price of original power wheels battery.

Thanks in advance for your help, and sorry for my English.

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  • \$\begingroup\$ Does not the laptop computer battery normally have protective circuits? \$\endgroup\$ – Mats Sep 9 '15 at 6:19
  • \$\begingroup\$ I’m not sure if those laptops batteries come with internal protection circuits I suppose I’ll have to check for that. \$\endgroup\$ – Enrike Barreto Sep 9 '15 at 7:02
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Having an amp rating HIGHER than the original battery is ideal. It will only try to deliver the car the current it needs. If the toy asks for more than the battery can offer, then you have a problem. However, you need to design the battery bank to safely fail when a cell dies, otherwise you can cause a cascade failure of venting and possibly igniting cells. A very bad thing. Here is an example of what I would do:

If we are using Panasonic 18650 3200mAh Li-ion batteries, then we know their specs are 3.6 nominal voltage and 10A max continuous current draw. First, calculate the bank characteristics. 10.8 / 3.6v ~= 3. We will need to use 3 batteries in series to pull the voltage up to spec. Max charge should be around 12.6v, likely nearly identical to the original battery. Assuming the old battery had a 30 amp fuse, we would need a minimum of 3 rows of 3 batteries to get the bank to the minimum requirements at nominal voltages. However, I would add 2 extra row for overhead. This would total 15 18650 batteries, 5 sets(in parrellel) of 3 batteries(stacked/in series). When charging the cells, you should charge them in parrellel. What I would do is make a separate set of terminals on the battery that connect to all of the batteries in parrellel. from these terminals, charge the battery at no more than 1 amp per cell at no more than 4.2 volts(for 18650 batteries, you need to find the nominal charge rate of the batteries you plan to use if you go another route). Charging batteries in parrellel will naturally balance, as long as those batteries are new batteries married to each other at birth. I have never trusted series charging because of all that can go wrong. This would give you a total of 5*3200 mAh for a total of 16000mAh and 173 Wh. This is simply an example to get you familiar with the calculations. A battery bank such as this would easily set you back USD $100. It is important to ensure each cell has overcurrent protection of an amperage lower than the battery is rated for(roughly 2 times the C rating). Without this, when a cell dies, it can cause an overcurrent cascade, and this is a very very bad thing.

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  • \$\begingroup\$ Thanks Richard for your quick response, I never considered the over current caused by a battery fail in the arrangement. In order to keep it simple, I never planned to disassemble the laptop batteries, that way I’ll take advantage of each charger protection circuits. In my crazy project I will use the battery packs as they come, my fear is to cause fire or worse if I put those on parallel and put them on that kind distress. I’m not sure if those laptops batteries come with internal current and over discharge protection circuits I suppose I’ll have to check for that. \$\endgroup\$ – Enrike Barreto Sep 9 '15 at 7:01
  • \$\begingroup\$ Laptop batteries usually do have overcurrent protection, but usually they provide their own electronics for this. Disassembling those batteries would require disconnecting the provided protection. It would be a great idea to keep them intact if you are uneasy about providing your own protection. \$\endgroup\$ – Richard Smith Sep 13 '15 at 0:48
  • \$\begingroup\$ But, still safe to put 4 laptop batteries in parallel? Will they cut the power when they reach low voltage? I need them to be in parallel to make enough current (10.4amp)... \$\endgroup\$ – Enrike Barreto Sep 23 '15 at 1:40
  • \$\begingroup\$ The batteries will not cut their own power, no. And draining them lower than spec(2.5v-3v) will damage them. It is perfectly safe to run them in parrellel, for you, but it is not safe(for the batteries) to keep draining them beyond their rated cut-off voltage. Parallel batteries will keep their voltage the same, so ideally you can't accidentally over discharge a battery by monitoring a set of them in parallel. The easiest way to prevent over discharge is to use a voltage cut-off relay. \$\endgroup\$ – Richard Smith Sep 25 '15 at 3:05

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