Float charging 2 batteries with one solar panel

[AndrewJ]

I have got a 5W micro solar panel with an inbuilt blocking diode that can ( allegedly ) be hooked up to a battery ( without needing a regulator ).
The specs recommend a min battery of 50Ahr
I have 2 SLA batteries 40/20Ahr and i want to use the unit to apply a float charge to keep the batteries topped up between use. I will recharge them using a proper charger, this idea is only to give them a daily float to prevent slow discharge over months of no use.
I know the 2 batteries have different "full" voltages, so i cant just parallel them.
I was thinking to use a common ground, and split the panel V+ into 2 leads using diodes in each leg so that i dont get cross charging between the batteries.
The panel puts out 17.4V at 290mA max
so if i do this, i will drop the max float voltage to say 16V, which is still above the "rated" float voltage, of 14.4, so i should be OK.
Can anyone see anything wrong with my logic????
ie is the 14.4V specified by the battery manufacturer a real limit if i am only putting in say a max of 100mA

Andrew

[glend (Glen)]

There is no reason you cannot parallel the batteries, they will just sit at the lowest full voltage of the weaker one. Ideally you should start with new batteries from the same product line and design, but it will work.
As far as dropping the panel output, which is often shown as open circuit values btw, the best approach in my opinion would be a simple, safe, and very cheap solar controller box, which you can find on ebay for about $10.
Good luck.
Btw, those 5W micro chargers are pretty useless, their output is very small. A small cheap controller box will allow you to use a proper solar panel to charge and it will quickly recharge any drain. Some solar controllers provide a fused output to the load side. Why connect a charger to recharge then between uses when they can be wired to charge constantly. MPCC controllers allow you to set float values or use default values. Output protection features will cutoff to prevent over discharge and thus protect the battery and extend its life. I never discharge my bank below 12.5V.

[AndrewJ]

Gday Glen

Btw, those 5W micro chargers are pretty useless, their output is very small.

Understood, but i have one, and all i want to do is utilise it to keep the batteries fresh between road trips ( which can be a long time between in Melb )
All my batteries/chargers etc have anderson connectors, so swapping from a proper mains charger to the float unit is 2 seconds work.


Andrew

[skysurfer]

Originally Posted by glend

Btw, those 5W micro chargers are pretty useless, their output is very small.

Well, currently Anker and RAVpower offer collapsible panels for a reasonable price. I ordered an Anker one of 5V/2A which works fine. I charge my cellphone and GoPro easily even on a slight cloudy day.

There should be 12V models available too.

http://www.outdoorgearlab.com/Solar-Charger-Reviews