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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