12V 2 Amp battery charger reducing

Thread Starter

homerdodd

Joined Feb 10, 2018
62
Good afternoon. I have a standard car battery charger, which when set on the lowest setting outputs 12Volts and 2amps, which is great for slow charging larger car batteries. However, I also have a much smaller motorcycle battery which will literally get warm or hot if charged with 2 amps or more. My question can I somehow reduce the 2 amp output to 1 amp or even .5 amp , which will then save me the money of buying a motorcycle charger ( I will if I have to). My first thought is a simple resistor pack connected inline between the chargers positive output cable and the motorcycle battery positive post. Just want to make sure I do this correctly, if possible. Is there another way (caps and/or resistors). Do you have any size recommendations ? Thanks !!
 
Last edited:

Reloadron

Joined Jan 15, 2015
5,456
My first suggestion is just go ahead an buy a Battery Tender, I have a few similar to this one and they work great. I have seen them as low as $18 USD. During the winters I leave one on my bike and one on my wife's truck out in the garage and anytime I want to start either they crank right over. Normally as a SLA (Sealed Lead Acid) battery charges the charge current will decrease based on an applied voltage. The fact that your two amp charger gets the batteries warm leads me to believe the applied voltage is higher than it should be. This is a good read on the subject. Anyway both of the tenders I have are 750 mA (3/4 Amp) and as I mentioned they work great and regulate the float rate on the batteries. My local Harley dealership sells the same thing but with a Harley logo on it for $49.95. :) My neighbor and friend also has his bike in our garage and using a different brand and same results. My bike and his both have a simple quick disconnect, Here is an example of a tender under $20 USD which includes all the hardware you need. Really, rather than mess with it just buy a tender and let it do its thing.

Ron
 

oz93666

Joined Sep 7, 2010
737
Your first thought is right ... you just need a resistor in the circuit ( I guess the charger has a built in Amp meter so you can see what's happening) ....

You need low ohms .... what do you have at hand? ... a long length of wire ... a car head lamp bulb ... an electric kettle ... all these have about the sort of resistance you are looking for , and can also dissipate the small amount of heat ....

A 10m length of house lighting wire would be about right ... 2 x 1mm squ ... you can put the two wires in parallel or in series, one of these combinations should probably do it.

If you want to do it properly get a bag of 1 ohm 5W resistors ... putting these in series or parallel gives a good range
 

MisterBill2

Joined Jan 23, 2018
6,090
Your first thought is right ... you just need a resistor in the circuit ( I guess the charger has a built in Amp meter so you can see what's happening) ....

You need low ohms .... what do you have at hand? ... a long length of wire ... a car head lamp bulb ... an electric kettle ... all these have about the sort of resistance you are looking for , and can also dissipate the small amount of heat ....

A 10m length of house lighting wire would be about right ... 2 x 1mm squ ... you can put the two wires in parallel or in series, one of these combinations should probably do it.

If you want to do it properly get a bag of 1 ohm 5W resistors ... putting these in series or parallel gives a good range
A chunk of wire will not provide a suitable resistance. If you just want a trickle charge put a 12 volt tail light bulb in series with one lead. Don't use the stop-light side as that will deliver too much current. My older charger has a 6 volt/12 volt switch and on the 6 volt side it does deliver less current into a 12 volt battery. That may also be useful.
 

Thread Starter

homerdodd

Joined Feb 10, 2018
62
A chunk of wire will not provide a suitable resistance. If you just want a trickle charge put a 12 volt tail light bulb in series with one lead. Don't use the stop-light side as that will deliver too much current. My older charger has a 6 volt/12 volt switch and on the 6 volt side it does deliver less current into a 12 volt battery. That may also be useful.
Thanks. That is exactly what I did. Soldered some leads to a single filament 12V lamp bulb, and installing inline between the positive battery post and the positive cable on the charger. It is working, without the case getting hot or even warm. The voltage has increased from about 9v base voltage to over 11v over about a three hour period. The lamp glows dim. I can seen the electrolyte in the individual cells slightly bubbling, indicating that cell is receiving a charge. Seems to be working good. Thanks.
 
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