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I bought a motomaster eliminator smart charger, it has reverse polarity and all that jazz, it says it's a 12 volt charger.

I am trying to charge 4 12 volt batteries for my electric bike. They are all wired together. they are soldered and I can't undo them because I don't have a solder gun. The charger didn't seem to work so I reversed the wires a few times to make sure my positive and negative were correct (black, and white wire).

The charger blew shortly after I plugged it into the wall. Is it the charger, or me. I can exchange it and get a new one, but what's the point if im going to just blow up another one?

Is this charger just not powerful enough to do what I want it to do?

Is it the charger? Or am I doing something wrong?

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    \$\begingroup\$ Are the batteries in series (48v) or parallel (12v), what is current is the charger rated for? What kind of batteries are they? Generally forcing a charger not made for specific batteries is a bad idea. \$\endgroup\$ – Jarrod Christman Sep 17 '15 at 21:02
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Your electric bike probably uses a 48V power source to operate its motor. Since you have 4 batteries, I'm assuming that your batteries are lead-acid. Why they are soldered together I have no clue because most often such batteries are connected in series with jumper cables. Assuming you have LA batteries that means you probably have 4 12V batteries that are connected in series--which = 48V (4 batts * 12V).

Before doing anything that I suggest below, make sure your batteries are 12V lead-acid batteries. If they are some other chemistry (e.g. li-ion batteries), do NOT follow the advice below.

To use the charger that you bought to charge your batteries (again, assuming they are LA batteries), you should borrow/buy a soldering iron & a multimeter. Use the soldering iron to separate the batteries & use the multimeter to verify the voltage of each battery before, during, & after charging each battery separately.

It is possible to charge all of your batteries at the same time using parallel connectivity, but I suggest you test & charge your batteries individually until your get your battery pack operational again--then you can get a 48V charger to simplify future charging.

Best regards!

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    \$\begingroup\$ I'd also suggest before starting that the OP familiarises himself with battery safety and the many Bad Things that can happen with batteries of all types and sizes if you get things wrong - a face full of hot acid can really ruin your day, mind you it does prevent you seeing your bike / house burning to the ground... \$\endgroup\$ – John U Sep 18 '15 at 17:04
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You have to know that NEVER batteries connect in PARALLEL, because it will damage them and discharge them rapidly.

When your charger is blow we can conclude these statements:

  1. Your battery packs are connected series and its voltage is greater than 12V. So when you connect them to charger, current flows throw batteries to charger and this will blow your charger.

I advice in the case of uncertainty you always use a high current diode like 1N5817 or 1N4007 to control current flow in series to your supposed current flow direction.

  1. Maybe you have used the wrong polarity for connecting charger to batteries. Then before any struggle, use a multimeter.

You have two options:

  1. Use another charger with higher voltage, e.g. 48V or 50V.

  2. Or unpack your batteries and charge them individually.

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    \$\begingroup\$ I'd suggest that most of the downvotes are because suggesting that you should never connect batteries in series, and the reason you provide, are not correct. You could never complete a circuit of batteries in series without a load. Connecting batteries in series is obviously extremely common from kids toys right up to the battery systems used in cars as a means to achieve a higher voltage than an individual battery. \$\endgroup\$ – Fuzz Sep 18 '15 at 11:58
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    \$\begingroup\$ Your advice for using a diode may be useful, however some battery chargers measure the battery voltage and set their charge current accordingly. Adding a diode will mean the battery voltage measured is incorrect (unless there is a seperate measurement wire, which is possible). I would suggest that you were right in suggesting that the battery voltage was higher than 12V (likely 48V as in other posts) and therefore would have damaged the charger. \$\endgroup\$ – Fuzz Sep 18 '15 at 12:00
  • \$\begingroup\$ yes Fuzz I was wondering how i write series instead of PARALLEL... \$\endgroup\$ – HOPE Sep 18 '15 at 13:43
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    \$\begingroup\$ That's still not exactly correct @Hope. Batteries are connected in parallel are also fine and extremely common way of adding battery capacity. Some care needs be taken so that you don't have batteries connected in parallel that have significantly different voltages. They will try to equalise each other and if the voltage difference is too high you can cause damage or overheat your connecting circuitry. There are a lot of ways around this, including connecting in parallel via a resistor first. I think what you mean is to connect then in parallel with reverse polarity. \$\endgroup\$ – Fuzz Sep 18 '15 at 13:59
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    \$\begingroup\$ Ok, it sounds good now. Paralleling have advantages and disadvantages as you explained completely. Now I can accept your opinion, not always emergency but when it's applicable. \$\endgroup\$ – HOPE Sep 18 '15 at 15:05

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