chargers Amps vs volts

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
I have this electric skateboard I built last year and the charger just died on me. It used to charge at 29v 1.5 amps, I have a charger laying around with the same connector that charges at 12v 2.5 amps. Would it be safe to use that charger / what would be the inferred effects in terms of charge time if I attempted it?
Thanks,
 

MrChips

Joined Oct 2, 2009
22,101
It is important to have the correct voltage and current capabilities of the charger.
But before we can prescribe the correct voltage and current, you need to tell us more about the batteries.
What type of battery chemistry, make, model?
How many batteries?
How are the batteries wired? Can you post a circuit schematic?
Are there any battery management electronics involved (i.e. intelligent charger circuitry)?
 

Reloadron

Joined Jan 15, 2015
5,667
What exactly is the battery? The battery type and voltage determine the charge needed. The 29 volt is a bit peculiar, was that open circuit or with the battery charging? To give a generic answer a charger rated at 12 volts will not charge a higher voltage battery like a 24 volt battery for example.

<EDIT> I see Mr. Chips beat me by a min. :) </EDIT>

Ron
 

spinnaker

Joined Oct 29, 2009
7,835
I have this electric skateboard I built last year and the charger just died on me. It used to charge at 29v 1.5 amps, I have a charger laying around with the same connector that charges at 12v 2.5 amps. Would it be safe to use that charger / what would be the inferred effects in terms of charge time if I attempted it?
Thanks,

Likely they are lithium batteries if it is a skateboard. Unless you want to risk burning your house down, use the charger designed for the skateboard.
 

Tonyr1084

Joined Sep 24, 2015
5,249
Aside from what has already been obviously stated by others, consider this: Power is the result of volts and amps. In DC circuits such as a battery charger your power will be rated in "Watts" In AC circuits it's rated in VA (or VoltAmps), meaning sort of the same thing as watts, but not the same thing because of the nature of alternating current. Since you're charging batteries, perhaps a little understanding of how things work will help you. Batteries have internal resistance that changes with the state of charge. I'm no battery expert so I'll leave the finer points to others. Nevertheless, lets consider the power you're putting into your batteries. Watts is the product of volts times amps. So if your charger is rated at 29 volts at 1.5 amps you're looking at a power factor of 29 x 1.5 = 43.5 watts. Simply choosing a charger who's voltage may be lower but amperage higher isn't going to give you more than you think. For instance, you propose using a 12 volt 2.5 amp charger. First, using the lower voltage just won't work for higher voltage batteries as others have already stated. Nevertheless, 12 x 2.5 = 30 watts. In the end, your power is less than if you use the correct charger.

Incidentally, 29 volts IS an odd voltage. So I'm figuring on 4 batteries at 7.2 volts, which comes to 28.8 volts. Pretty darn close to 29 volts as you say your charger is rated for. So maybe that is the right charger. Still, if you don't provide 28.8 volts then your batteries will not charge. It's not the amperage that charges them it's the potential voltage that does. By "Potential Voltage" I mean the rated battery voltage. You can have a battery with a potential voltage of only 24 volts, which may mean, in the case of your batteries, an undercharged or discharged battery. If you don't provide sufficient voltage then your potential will not go up. And to charge a 28.8 volt battery you'll need more than 28.8 volts. Since I don't know what type of batteries you have I can't say for certain what the proper charge voltage is.

On top of that, there's the battery chemistry. Some batteries can take a higher charge current (amperage) than other types. If you charge the wrong type of battery too fast it can explode. And that right there is the number one reason why you should use only the correct charger for your batteries. Simply choosing a charger with higher amperage isn't the correct answer. It needs to be the proper voltage and the proper amperage. Change those up and you can damage or destroy your batteries.

I'm also thinking that you might not have four 7.2 volt batteries, you might have only three 9.4 volt batteries (or whatever the correct 9 point something is) batteries. 9.4 x 3 = 28.2 volts. So I'm confident you have either 3 or 4 batteries in series. Series means one connected to the next, positive to negative of the next battery, that batterie's positive is connected to the next. The voltages add up. If you were to wire them in parallel, where all the positives are wired together and all the negative wires are wired together you'll still have the same voltage but a larger capacity, meaning they will last longer.

Whatever voltage your batteries are and whatever voltage your motor drive requires, don't try and change that either. Providing the incorrect voltage COULD burn out your motor.

All of this is fundamental to understanding how batteries work and how to charge them. It's also worth noting that all I can do is guess at what you have until you provide us with more concrete information like the exact type of batteries, the battery charger model number if you have it and anything else important to your system. You say it's home built. Good for you. Hopefully it was done with someone to help you do it right, as it seems (at least to me) that you may not fully understand what you are doing. Otherwise you would not be asking if you could use a 12 volt battery charger to charge a 29 volt battery.

Get back to us with as much information as you can provide. Even if you give us information that is unrelated, at least we can pick through the important stuff and give you the best answer you can get. The people here are really sharp, and they will give you the right answer.
 

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
Than you all for the replies and I believe I'm obligated to fill in the missing information just too make it clear. The battery is a Chinese made lithium battery and it is composed of what looks like 7 or 8 3.7v cells definitely wired in series but have some sort of internal circuit for charging(to keep the cells balanced). I can tel this because there are two leads from the battery, one is the charging lead which is a fairly thin gauge wire and the other is the output lead which is of a much thicker gauge. So do you guys think it would be safe for me to use my charger on it?

side note: Surprisingly no i'm not trying to burn my house down, I just wanted to save 40 bucks if I could. ;)
 

Tonyr1084

Joined Sep 24, 2015
5,249
Each 3.7 battery may have its own charge controller circuit, but the overall is still 29 volts. You simply can not use a 12 volt charger to charge a 29 volt battery. One more thing you said that scares the heck out of me is that the batteries are Chinese batteries. I'm not knocking things made in China, but I am very skeptical when it comes to electronics and electronic goods that come from China. There's a reason why they can sell something for 99¢ that everybody else sells for $6.49 - it's cheap. Cheaply made. Without all the quality control. Without all the safety and reliability that should go along with it.

I might buy Chinese chips, but only if it's a junk project. Something I don't care if it fails soon. But with Lithium batteries - I draw a very solid and bold line. I simply don't like unexpected fires. Expected fires - that's something else. But the last thing I want is to have my electronic project go up in flames simply because I made a bad choice.

The ONLY way you can use a 12 volt charger is if you disassembled your batteries into 12 volt segments. And I'd strongly advise against that as well. Unless your batteries each have individual wires for both positive and negative.

In short, I think you need to give up on the idea of charging your 29 volt battery with a 12 volt charger. But I WILL say this: You can go ahead and try. Charging a 29 volt battery with a 12 volt charger WON'T put your batteries at risk. Nor will it put your home, property or life at risk. It'll simply do nothing. Well, maybe it will discharge your batteries.
 
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