Primitive 12V battery portable charger.

Thread Starter

Sergio34

Joined Nov 10, 2018
22
Hello, I need to make something portable that will give to my motorcycle battery some topping up charge when not in use for days. As it is difficult to remove the battery and no mains supply is nearby I already have a solar charger but it works only when it is sunny and is not often. I know what I am proposing in the attached file is primitive, I do not intend to leave it unattended. In a few words connecting two small 12V batteries in series to give me 24V and that to the 12V motorcycle battery through a 25watt 10Ω resistor. Considering the motorcycle battery can accept up to 4A in fast charge 1.2A should be fine. I know that engine's alternator charges the battery at about 0.9A. Also I know that when engine is charging the battery voltage can reach up to 14-15V. So the first time I will use this configuration (if you approve...) I will keep an eye to the voltage and when it reaches 14.5V I will disconnect it. If that happens in one hour for example next time I will leave it for 30'-45' to be safe.
 

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dendad

Joined Feb 20, 2016
3,071
If your battery cannot hold a charge for a few days, you need to replace it. The battery should stay charged for weeks, if not a couple of months.
A solar panel will be fine. even a quite small one will keep the battery topped up if the battery is ok.
So, get your ride a new battery.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
22
If your battery cannot hold a charge for a few days, you need to replace it. The battery should stay charged for weeks, if not a couple of months.
A solar panel will be fine. even a quite small one will keep the battery topped up if the battery is ok.
So, get your ride a new battery.
Thanks, but I am asking about the functionality of my circuit, replacing the battery is not what I asked for.
 

crutschow

Joined Mar 14, 2008
23,507
The circuit is okay but it's rather crude.
Adding the common LM317 regulator would be a better option option.
It inherently limits the current to about 1.5A.
You adjust the output to a float voltage of about 13.6V so you don't have to monitor it.
It does need to be on a heatsink to dissipate the approximately 18W maximum power.
The heatsink would need a thermal resistance to air of no more than 4°C/W.

For higher efficiency (around 90%), you could also use a DC-DC buck-boost converter like this, to generate the charging voltage for the motorcycle battery from a single external battery.
It regulates the voltage/current (both adjustable) so you don't have to monitor the battery.
As above, a float charge of about 13.6V should work.
 
Last edited:

Thread Starter

Sergio34

Joined Nov 10, 2018
22
The circuit is okay but it's rather crude.
Adding the common LM317 regulator would be a better option option.
It inherently limits the current to about 1.5A.
You adjust the output to a float voltage of about 13.6V so you don't have to monitor it.
It does need to be on a heatsink to dissipate the approximately 18W maximum power.
The heatsink would need a thermal resistance to air of no more than 4°C/W.

For higher efficiency (around 90%), you could also use a DC-DC buck-boost converter like this, to generate the charging voltage for the motorcycle battery from a single external battery.
It regulates the voltage/current so you don't have to monitor the battery.
As above, a float charge of about 13.6V should work.
Thanks a lot, very helpful
 
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