[Neddy]

Many people think what is required here is a Trickle charger, but that is not the best option. Trickle chargers generally supply a fixed current - regardless of how much or how little the batteries may need. A far better system is to use a Float charger. These maintain batteries at their recommended Float voltage, varying the current as necessary. Such a charger can be run from Mains power or your House batteries, but a really good option is to utilise your existing solar panels. It doesn't take a lot of power to maintain batteries at their float voltage so the Float charger can use the same solar panels as the existing House battery solar controller.

A suitable "boost" power supply module like this one only costs about $6. It will take your solar panel output and step it up to whatever Float voltage is recommended for your specific Starter batteries (probably around 27.6 volts). The power supply is set to the desired Float voltage with the on-board potentiometer you can see on the right, using a digital multimeter.


https://www.aliexpress.com/item/High-Qu ... 9a07fb5492
A 10 amp diode is fitted to protect the charger from incorrect (reversed) solar panel connection, and another 10 amp diode is used to protect the charger when the battery voltage rises above the set Float voltage (eg whenever the engine is running). No fuse is necessary on the input side because the solar panels are a current-limited power source, but fitting a 10 amp fuse on the output side would be a good idea.



You could fit a switch to control this Float charger if you wanted to, but it would be quite safe left on all the time. With it set to the correct voltage the battery cannot be overcharged. An ammeter could be fitted to show the charge current and/or perhaps a voltmeter to show the working voltage and make setup even easier.
The Float voltage can be varied, of course. It could be set lower than 27.6 volts to lower the charge rate and thus use less solar power or it could be set higher than 27.6 volts to charge the Starter batteries faster, at a higher current - should you ever want/need to do this.

Neville.

[Nut17]

That is an excellent solution Neville I wish I knew you when I faced the conundrum of the mix of 24 volt and 12 volt requirements when I built my spray truck. I can see your suggested device being perfect for Mark's situation.

Cheers
Chris

[96coaster]

The link below is to what appears to be same item on Ebay, but while it may be slightly cheaper, the copper winding's around the core look a bit of a mess, and certainly not as neat as the AliExpress item, guess you get what you pay for.


http://www.ebay.com/itm/150W-DC-DC-Boos ... SwcLxYDvpi

[Mark]

Hey Neville, that looks like something even I could manage to put together (and well worthwhile - even at that exhorbitant cost)

A couple of questions, though (inevitably )
1. You may recall my solar panels are 9V. I see this unit has a minimum required input of 10V. Is that close enough, or should I look for a unit with a lower input voltage?
(or, on further thinking, could I use the "load" output of my solar controller to provide (whatever it is 13.7 - 14.3V)?)
2. Given that the output voltage is variable, I guess if I mount the unit near the existing controller (convenient), then the distance and gauge of cable to the starter batteries can be compensated for by the voltage adjustment on the unit?
3. The red in your diagram is positive?

I appreciate the time you spent putting this very useful post together, for me - and I suspect, quite a few others.
Have you yet negotiated a finder's fee facility with AliExpress?

[Neddy]

"You may recall my solar panels are 9V. I see this unit has a minimum required input of 10V."
I knew that your panels were 9v but thought that you were running them in series (as depicted in my wiring diagram) and getting 18 volts. Have I got that wrong and you are in fact running them in parallel?

"Is that close enough, or should I look for a unit with a lower input voltage?"
No, it's not close enough. 9v is too low for that particular model.
If you do have your panels in parallel rather than series, here is a power module with a 5v minimum input that would do the job well. It also offers variable current limitation - nice to have. Unfortunately the price skyrockets with this model, and a much more substantial financial investment is called for - more than $15, in fact. Add to that the cost of diodes (at, say, 20 for nearly $2) and you can see how the overall cost of such a project quickly escalates - even before you add in the cost of any cable involved. Admittedly this need only be of fairly light gauge, but even so.......
https://www.aliexpress.com/item/CFSUNBI ... f27710d9e4

"Could I use the "load" output of my solar controller to provide (whatever it is 13.7 - 14.3V)?"
That would work, but wouldn't it mean that it would draw power from your House battery if there wasn't much sun? If so, that is not something you want to do.

"Given that the output voltage is variable, I guess if I mount the unit near the existing controller (convenient), then the distance and gauge of cable to the starter batteries can be compensated for by the voltage adjustment on the unit?"
Right, it could be. The currents involved are likely to be relatively low, though, so I doubt that this advantage is of any great importance.

"The red in your diagram is positive?"
It is supposed to be, but didn't come out very clearly. I think that my increasing the line width would help.
Next time.

Neville.

[Mark]

You may recall my solar panels are 9V. I see this unit has a minimum required input of 10V."
I knew that your panels were 9v but thought that you were running them in series (as depicted in my wiring diagram) and getting 18 volts. Have I got that wrong and you are in fact running them in parallel?"
Yes, connected in parallel. It is heavy overcast today and the MMPT controller is telling me the input voltage is 10 - 10.2V. My DMM is showing 8.9 - 10.3V

"Is that close enough, or should I look for a unit with a lower input voltage?"
No, it's not close enough. 9v is too low for that particular model.
If you do have your panels in parallel rather than series, here is a power module with a 5v minimum input that would do the job well. It also offers variable current limitation - nice to have. Unfortunately the price skyrockets with this model, and a much more substantial financial investment is called for - more than $15, in fact. Add to that the cost of diodes (at, say, 20 for nearly $2) and you can see how the overall cost of such a project quickly escalates - even before you add in the cost of any cable involved. Admittedly this need only be of fairly light gauge, but even so.......
https://www.aliexpress.com/item/CFSUNBI ... f27710d9e4
That's a nice looking unit but "escalates" he says. I believe a more precise term would be "blows-out". Looks like I'll have to wait for the next pension payout (and hope it doesn't clash with the need for a Southern Comfort top-up)

Yes, I see what you mean about using the "load" from the controller.

Would a grouping of wires within a CAT5 cable be too light for the likely current for connecting between the unit and the battery? It's just that CAT5 is nice to run and I have lots of it - so might still be able to afford the SC top-up.

[BUSRSQ]

TLDR And it all looks pretty awesome, one other option is if you are running an inverter is to plug in a 24v battery charger and use that to trickle charge your start battery's

[Neddy]

"Would a grouping of wires within a CAT5 cable be too light for the likely current for connecting between the unit and the battery?

I think that running two sets of 4 wires bundled together would be OK. We do not really know what current levels to expect but the current limiting control on the more expensive unit would mean that it could be capped at pretty much any level you like. Initially it could be set at a lowish figure of say 1 or 2 amps, for example.

Neville.