How to charge an AGM trolling motor battery whilst on-board

[affinite]

When at anchor I charge my 520Ahr house bank using a 400W PV panel and Victron 100/30 MPPT charger (which works well in Greece)
When connected to shore power I have a Sterling Ultra 50A charger and when underway on engine I charge using a Sterling 50A A-B charger.
(These also charge my 200Ahr Thruster bank and 80Ahr Start battery through a Sterling ProSplit charge splitter)

This season I am experimenting with an electric trolling motor on the tender and Im powering it using a 60Ahr AGM deep cycle battery.
I would like to charge it during daylight using the MPPT controller but I dont know how to best connect it.
Can I just connect it in parallel with the existing house bank using a VSR?
(I suspect not as its AGM and the house bank is FLA with different charging profiles)

Is a B-B charger the only way forward.

Any advice or suggestions welcome.

 

[Spirit (of Glenans)]

The little (dangerous ) knowledge I have gleaned on this forum would lead me to suggest that your AGM battery needs a different charging regime to your existing ones and would be best served by a B-B setup. Undoubtedly someone more knowledgeable than me will be along shortly with more detailed advice.

 

[LittleSister]

Years ago I had bought a boat that had AGM engine start and domestic batteries, and a simple voltage regulated alternator for charging. I read on YBW that one should charge AGM batteries at a different voltage to Flooded Lead Acid ones (I see my battery charger charges AGMs at 14.7v compared to the standard 14.4v) and never mix the two. I contacted Merlin Electronics, who had supplied much of the switchgear etc. (and likely the batteries) for the boat, to ask if I needed to have some different arrangement. They confidently told me that I didn't - the AGMs would be absolutely fine with my standard arrangement, and so it proved over many years. (Perhaps I could have got slightly more charge in at the higher voltage, but it was never an issue.)

It may be that a fancy charger that bulk charges a depleted battery at a higher rate would not suit an AGM, I don't know, but if your MPPT controller is just putting out a normal fixed voltage it will, in my experience, be fine.

 

[VicS]

When at anchor I charge my 520Ahr house bank using a 400W PV panel and Victron 100/30 MPPT charger (which works well in Greece)
When connected to shore power I have a Sterling Ultra 50A charger and when underway on engine I charge using a Sterling 50A A-B charger.
(These also charge my 200Ahr Thruster bank and 80Ahr Start battery through a Sterling ProSplit charge splitter)

This season I am experimenting with an electric trolling motor on the tender and Im powering it using a 60Ahr AGM deep cycle battery.
I would like to charge it during daylight using the MPPT controller but I dont know how to best connect it.
Can I just connect it in parallel with the existing house bank using a VSR?
(I suspect not as its AGM and the house bank is FLA with different charging profiles)

Is a B-B charger the only way forward.

Any advice or suggestions welcome.

Your Victron MPPT controller is fully programmable for different battery types. You could opt for charging both AGM and FLA batteries from the solar controller using a VSR ( eg a Victron Cyrix) by setting each stage of the charging algorithm to the lower of the two alternative voltages.
ITYWF that AGMs should not be subjected to the "equalising" stage . That will have to be turned off.

It might be interesting to compare the cost of a Cyrix VSR with the price of a B2B charger.

What make would a B2B be ? Not every one is all that enthusiastic about Sterling products but you are presumably happy with the ones you have.

 

[affinite]

I just checked and bulk charging voltage for both my FLA bank and the new AGM are exactly the same at 14.82v so maybe the Victron Cyrix is a goer

 

[William_H]

When charging from a solar and MPPT source. Much depends on the size of the solar array amount of sunlight and battery size. So in many cases with limited charge capability the MPPT will be trying to provide the desired bulk charge voltage but actually failing to reach that voltage due to lack of power from the solar panel.
So in practice charge profiles mean very little when full charge going in to float mode is never achieved.
If this is the case and you should confirm by actual monitoring during the day then yes just connect AGM in parallel with FLA and monitor voltage. It may be that you would get more charge in to AGM by redirecting the MPPT from FLA to AGM alone. You may want to set up MPPT fro AGM but as argued this may nopt matter. ol'will

 

[PaulRainbow]

I just checked and bulk charging voltage for both my FLA bank and the new AGM are exactly the same at 14.82v so maybe the Victron Cyrix is a goer

What are the FLA batteries ? (makes and models please)

 

[affinite]

House bank is comprised 4 x Trojan 31 XHS (4 x 130Ahrs)
Last summer I installed the new PV panel so I did observe the charging profile carefully. using the Victron App
The MPPT did run a full Bulk/Absorption/Float charge every day reaching float after 3-4 hours
(Boat is in Southern Greece so sunny pretty much every day)

 

[PaulRainbow]

House bank is comprised 4 x Trojan 31 XHS (4 x 130Ahrs)
Last summer I installed the new PV panel so I did observe the charging profile carefully. using the Victron App
The MPPT did run a full Bulk/Absorption/Float charge every day reaching float after 3-4 hours
(Boat is in Southern Greece so sunny pretty much every day)

Given your location and the size of your solar array, you'll be giving the AGM battery a hard time if you use a VSR. I'd suggest a Victron B2B charger, that'll charge the AGM battery from all charging sources too.

For example, this costs little more than a decent VSR: Victron Energy Orion-Tr 12/12V 9A (110W) Isolated DC-DC Converter - ORI121210110R

 

[VicS]

Given your location and the size of your solar array, you'll be giving the AGM battery a hard time if you use a VSR. I'd suggest a Victron B2B charger, that'll charge the AGM battery from all charging sources too.

For example, this costs little more than a decent VSR: Victron Energy Orion-Tr 12/12V 9A (110W) Isolated DC-DC Converter - ORI121210110R

but Victron say they are not intended for battery charging and advise the use of an Orion Tr Smart DC-DC charger.

These cost considerably more. About £160 upwards

 

[William_H]

but Victron say they are not intended for battery charging and advise the use of an Orion Tr Smart DC-DC charger.

These cost considerably more. About £160 upwards

As I read the blurb can be used for battery charging. Of course only as a regulated supply type charger (not smart charger) so with diminishing charge current with battery charge. Perhaps not best for OP requirements. It seems OP solar charging is able to bring batteries to full charge and so go top correct bulk charge trickle charge regime asset for FLA so may not suit AGM in parallel or on the same charger without adjusting settings. Of course he could just parallel the outboard battery fort a limited period monitoring charge current. Perhaps using a series resistor if charge current seems too high. ol'will

 

[PaulRainbow]

but Victron say they are not intended for battery charging and advise the use of an Orion Tr Smart DC-DC charger.

These cost considerably more. About £160 upwards

My mistake, some DC-DC converters came up in the search results for DC-DC chargers and i didn't notice. Should have been (as you say) :

Victron Energy Orion-Tr Smart 12/12V 18A (220W) Isolated DC-DC Charger - ORI121222120

 

[PaulRainbow]

As I read the blurb can be used for battery charging. Of course only as a regulated supply type charger (not smart charger) so with diminishing charge current with battery charge. Perhaps not best for OP requirements. It seems OP solar charging is able to bring batteries to full charge and so go top correct bulk charge trickle charge regime asset for FLA so may not suit AGM in parallel or on the same charger without adjusting settings. Of course he could just parallel the outboard battery fort a limited period monitoring charge current. Perhaps using a series resistor if charge current seems too high. ol'will

The charging regime for the OPs Trojan batteries is suitable for his AGM battery, but charging in parallel with a large solar array in Greece might not be best for the AGM, especially if he cycles the Trojan daily.

He could parallel the AGM and manually check it for charging as a short term thing while he tests the viability of the system, but that's not really a satisfactory long term arrangement, IMO.

Limiting charge current isn't necessary, the battery will self limit current, it's long term exposure to bulk voltage i'm thinking about, although that might not be an issue given the battery regimes. I've head of standard FLA batteries getting cooked in Med Sun with solar arrays set to charge deep cycle batteries on a daily cycle, but as the AGM needs those higher voltages it might not even be an issue. Might be worth paralleling the battery manually and monitoring for a few days, but disconnect if the boat is left for a while.

 

[affinite]

Thanks for the great advice chaps
I'll start experimenting with manual parallel connection when I get to the boat on Tuesday and I'll ask the admiral to put the Victron Orion Smart on my birthday present list.
Hang on. its my birthday on 29th May mmmmm