Charging a smaller capacity SLA battery from a larger bank.

[alesserfate]

Hey folks,

I am currently running a 150Ah 12V AGM-Gell Cell battery bank (yes, fairly small) with a MPPT solar controller which feeds 300W of solar to about 26 amp of 12V DC on a good day to charge / maintain the bank. I also have a 12V AGM 50Ah standalone SLA battery which I use to power the 12V trolling motor on my boat.

My question: when I need to charge the 12V 50Ah SLA AGM battery, can I just connect it to the main bus of my 150Ah solar bank and let it sit and charge up ? How can I tell when its full? Or is there a better way ? I also have a Noco 110v smart charger but it takes a lot of energy to run off my inverter. If I use the 110v charger with the inverter, by the time its done charging, I won't have enough in my 150Ah bank left to run my fridge through the night without sunlight.

Thanks in advance.

 

[kellys_eye]

You normally use a split-charging diode arrangement - the solar feed goes to two diodes (suitably rated for amperage etc) and each diode cathode then connects to the individual battery banks.

You also (but not always) have to compensate for the diode forward volt drop.

 

[alesserfate]

You normally use a split-charging diode arrangement - the solar feed goes to two diodes (suitably rated for amperage etc) and each diode cathode then connects to the individual battery banks.

You also (but not always) have to compensate for the diode forward volt drop.

Thanks for your reply.

The solar is not an issue, I have an MPPT controller. I was just describing my setup in its entirety.

Maybe to keep it simple I should just put: I need to charge my portable 12V 50Ah battery from my 12V 150Ah stationary battery bank. What's the best way to do it ?

 

[kellys_eye]

Via a boost charge method. The battery requires around 14V to charge and a battery that is already charged (just off the charger) will only be around 13.8V, dropping quite quickly thereafter.

Use a simple boost converter to power a lead-acid charging circuit.

 

[WHONOES]

As a general rule, you should limit the charge current for a Sealed Lead Acid battery to no more than 25% of capacity. If you intend to charge the smaller capacity battery in isolation you will need to limit the charge current to 12.5A or less.
To establish when your batteries are fully charged, put a dc ammeter in series with either the + or - charge lead. Then when the current drops to a few 10's of mA (or maybe 100mA or so for the larger one) it is full up.

 

[alesserfate]

Thanks guys.. I understand what both of you mean. I'll have to figure out a practical way to set it up.