An unusual 12V (starter) battery question

[maingate]

I have a spare 12W panel (previously used on a boat) that I thought about fitting for that purpose, but the only practical place to mount it would be on the camper so another connector to plug and unplug and some fairly tortuous wiring. What's wrong with the maintenance charger to keep the starter battery maintained from the aux battery that is already being maintained by the solar?

I am thinking of making it as a "portable" unit, only fitted when laid up and removed when in actual camping use.

I have used a 9W solar panel to maintain spare batteries in my shed. It works fine and was at one time in the cab window to keep the engine battery topped up. Just fit crocodile clips and connect the 12W to the engine battery. End of problem (for the summer and autumn at least).

 

[merl]

n spite of the blurb telling us this is some complex electronics, externally it tests as a pair of diodes (using the diode test function on the Fluke multimeter) but with a very low forward volt drop.

If the forward voltage drop measures less than 250mv then it'll be an active device utilising FETs as pass devices, thinking about it it almost certainly is that because it needs a separate power supply and not simply the input/output terminals. I assume you tested the volt drop while it was still connected to the supply voltage?

 

[ProDave]

I have made my little maintenance charger and connected it. The DC clamp meter seems to suggest it is giving the starter battery a gentle 0.3A charge.

I will check it after 24 hours to see how the starter and aux battery voltages are doing.

I have made it such that it can be quickly unplugged. It is really only needed when laid up while in camper mode.

 

[ProDave]

So after less than 24 hours in use the maintenance charger has lifted the start battery voltage by about 0.4V and the initial drop in voltage of the aux battery when the maintenance charger was connected that too has risen closer to what it was.

So it looks to be doing the job and all 3 batteries should remain topped up from the solar while laid up.

Connecting it was a bit of a puzzle,. I wanted it to be removable easily. I looked at just big croc clips, but too many bits of grounded metal close to the battery terminals that made that potentially dangerous (see what I did there) So I soldered a 4mm banana socket to a large ring terminal and heat shrunk the assemble. One of those on each batteries positive terminal. Safe and shrouded when not in use. And the maintenance charger has two fly leads each with a 4mm plug on the end.

 

[merl]

So after less than 24 hours in use the maintenance charger has lifted the start battery voltage by about 0.4V and the initial drop in voltage of the aux battery when the maintenance charger was connected that too has risen closer to what it was.

So it looks to be doing the job and all 3 batteries should remain topped up from the solar while laid up.

Connecting it was a bit of a puzzle,. I wanted it to be removable easily. I looked at just big croc clips, but too many bits of grounded metal close to the battery terminals that made that potentially dangerous (see what I did there) So I soldered a 4mm banana socket to a large ring terminal and heat shrunk the assemble. One of those on each batteries positive terminal. Safe and shrouded when not in use. And the maintenance charger has two fly leads each with a 4mm plug on the end.

Mine is connected permanently, it's best to keep the starter battery a fully charged as possible to prevent sulphation and having it permanently connected helps in that respect.

 

[trevskoda]

Mine is connected permanently, it's best to keep the starter battery a fully charged as possible to prevent sulphation and having it permanently connected helps in that respect.

Same here, 10ah smart numax charges all the batteries with a flick of a switch., only required in depth of winter when solar just dont cut it.

 

[Thistle]

@ProDave
A couple of observations
First do not rely heavily on those volt meters unless they are completely isolated from any other loads and wired to sense the voltage directly across the battery terminals.

It is very common for these things to be on a circuit shared with other loads, end result voltage drop affects results

Second never assess a battery voltage in situ, you need to do what’s called a resting voltage test, disconnect it charge to a true 100% rest fir an hour check voltage e.g 12.9v rest for 24 hours check voltage a good battery will hardly change if it does by 2-300mv or so then that’s suspicious so would anything below 12.6v