Recharging trolling motor battery from main battery bank

[Jaakko]

Hi all,

I have a problem similar to what was discussed here, but I'm still looking for answer to the original concern, which wasn't quite addressed:

I have a 2x180Ah 12V sealed lead acid battery bank at my cabin, which is kept mostly fully charged by an MPPT charger and 2x250W panels. I also have a mobile 80Ah AGM battery that is used with a trolling motor on a boat, but unlike the boat battery in the referred thread, mine often drains quite low. So I would like to re-charge this boat battery from the main bank.

Ideally I would like to be able to just snap the boat battery parallel to the main battery bank, but I'm afraid with different battery types, different capacities and different charge levels, bad things could happen, right?

I wouldn't mind the charging taking it's time, so is it possible to limit the DC-DC charge distribution rate somehow (maybe to 6A or so)? DC-DC chargers also seem to exist, but with a big price tag - is that the only way to go?

So far the only safe solution I've come up with is to use a regular 230VAC-to-12VDC battery charger through an inverter (since I happen to have both) to achieve what I want, but it feels silly having to do the AC step inbetween.

Local solar panel retailer suggested me that easiest and cheapest would probably be to just buy a separate panel and simple charger dedicated for the boat battery, but I'd really like to benefit from the existing charge at the main bank, too, in case I'd need to get the the boat battery recharged overnight for example.

Thanks in advance for any ideas!

Jaakko

[inetdog]

If the trolling battery is very low and the main bank is fully charged, the current that starts flowing as soon as you connect the two will:
1. Make a big spark.
2. Probably damage the small battery,
3. Possibly overheat the wires you use to connect them.

So, you really need to have some sort of voltage and current control device in between. Unfortunately a simple CC will not do the job when the input and output voltages are so close (or identical).
Other than a DC-DC supply with current limiting on its output, or a simple DIY current limiter circuit built with an OP Amp or two and a FET, the idea of using an AC charger running off your inverter is about all that is left.
You did state that you wanted to be able to use the battery bank rather than being limited to charging only during the day. The critical point is to find out how fast you can safely charge the trolling battery. If it is a standard FLA battery you should plan on it taking at least 12 hours to fully recharge. Anything faster than that would require too high an initial current for the small battery to tolerate.

[Sunking]

Ideally I would like to be able to just snap the boat battery parallel to the main battery bank, but I'm afraid with different battery types, different capacities and different charge levels, bad things could happen, right?

Correct but not for the reasons you are thinking. Batteries have almost unlimited current capacity in this application. If you were to connect a discharged battery in parallel with a fully charged battery current would surge to thousands of amps flowing. By no means of exaggeration very possible you would have an explosion of both battery acid and plasma from vaporized wiring. It would be a very bad hair day indeed.

I wouldn't mind the charging taking it's time, so is it possible to limit the DC-DC charge distribution rate somehow (maybe to 6A or so)? DC-DC chargers also seem to exist, but with a big price tag - is that the only way to go?Jaakko

No you got it from all the possibilities you mentioned. If you could convince me you had some practical electrical experience I know of another way that is cheap, but you would have to know exactly what you are doing because one mistake and you have a Bad Hair Day.

One alternative, still a bit pricey, but get a Hobby Charger. By that I mean the ones used to charged Remote Control Airplanes and Cars with. They are made to charge any kind of battery and programable an most can charge a 1.2 to 36 volt battery from a 12 volt source. Something like this.

[bcroe]

Hi all,

I have a problem similar to what was discussed here, but I'm still looking for answer to the original concern, which wasn't quite addressed:

I have a 2x180Ah 12V sealed lead acid battery bank at my cabin, which is kept mostly fully charged by an MPPT charger and 2x250W panels. I also have a mobile 80Ah AGM battery that is used with a trolling motor on a boat, but unlike the boat battery in the referred thread, mine often drains quite low. So I would like to re-charge this boat battery from the main bank.

Ideally I would like to be able to just snap the boat battery parallel to the main battery bank, but I'm afraid with different battery types, different capacities and different charge levels, bad things could happen, right?

I wouldn't mind the charging taking it's time, so is it possible to limit the DC-DC charge distribution rate somehow (maybe to 6A or so)? DC-DC chargers also seem to exist, but with a big price tag - is that the only way to go?

So far the only safe solution I've come up with is to use a regular 230VAC-to-12VDC battery charger through an inverter (since I happen to have both) to achieve what I want, but it feels silly having to do the AC step inbetween.

Local solar panel retailer suggested me that easiest and cheapest would probably be to just buy a separate panel and simple charger dedicated for the boat battery, but I'd really like to benefit from the existing charge at the main bank, too, in case I'd need to get the the boat battery recharged overnight for example. Jaakko

Don't just connect the batteries together. A DC to DC control is the elegant way to solve the
problem, sorry its expensive. Building a good one is somewhat complex. The inverter to charger
scheme certainly is inefficient. There is one thing you could try. Connect a 12V bulb from the full
battery + to the discharged battery +, negatives together. The bulb will limit peak current (or burn
out if one battery is reversed). As charge comes up the bulb resistance will drop and tend to keep
some current going (bulb resistance can vary 20:1). By some experimentation with size bulb (or bulbs
in parallel), you may be able to get a decent part of the charge level where you want it, use a timer
of some sort to cut off or tell you to disconnect. This will vary with the state of the source batteries
and not cover the complete charge cycle, but its cheap. Bruce Roe

[Mike90250]

First, realize that there is a lot of potential for a "bad hair day" here. (skip one step, one wrong wire.....)

My suggestion - Let the solar charge up your house batteries, when they are full enough, shut down the Charge Controller, and rewire it to the trolling battery, and charge it from the PV array. If it's too low of voltage for the charge controller to work, you will need to use a couple (or 4) 12V headlight bulbs (55W @ 12V = 4 amps each) to gently boost it up enough to allow the charge controller to complete the task. You could even buy another PV panel.

[thastinger]

do you not have an inverter that powers the cabin?

You state that you have reserve capacity in the cabin power arrangement, so just wait for a nice sunny day and charge the trolling batt from a standard battery charger via the inverter outlet.

[PNjunction]

Sunking's idea isn't too expensive - while a hobby charger looks intimidating at first glance, it would be pretty easy to operate since you are only dealing with a two-terminal battery, and thus no balance leads or balance charging etc. Just set your lead-acid absorb to your desired voltage and the rest of the defaults would be fine. Of course this would drain your main bank.

Mike also has a great idea - why not get a simple setup going like a 100 watt panel and a pwm controller for that smaller batt?

[inetdog]

S
Mike also has a great idea - why not get a simple setup going like a 100 watt panel and a pwm controller for that smaller batt?

Maybe because it would not charge the trolling battery as fast and also could not charge the trolling battery overnight between daily uses. The OP stated that as one reason for wanting to be able to charge from the main bank.
Kind of like the charging of a plug-in EV only being available close to local noon can really cramp travel plans.

[jony101]

I charge my spare agms and even the starting battery from the house battery through one of the hobby triton chargers it maxes out at about 7 amps on the triton 2, but I charge overnight if I have to. The older tritons you can find cheap, i paid 30 dollars for mine and charge everything with them, better than a trickle charger. Your 80 ah battery when depleted would probably need 50 amps added back, at 5 amps it would take about 10 hours, some of the venom hobby chargers can handle 25 amps.

The triton has an lcd screen that shows the voltage / amps / total amps delivered. sometime i watch the lcd, say the battery is at 12.1 volts, and you have it set at 5 amps the amps start out slowly then it gets up to 5 amps and the voltage starts to slowly climb, when it reaches 14.4 it stays there for a while and you slowly see the amps start to drop, this takes hours. According to the manual, when the amps drop to under 100 milliamps thats when the charger stops, it seems to treat all my batteries that i put on it very good. So good I bought 2 chargers to have a spare.

Before I also wondered if it was possible to charge a 12 volt battery from another 12 volt battery, since I knew you need to have at least 14 volts to charge a 12 volt battery. But these hobby chargers have circuitry that raises the voltage.

[PNjunction]

Maybe because it would not charge the trolling battery as fast and also could not charge the trolling battery overnight between daily uses. The OP stated that as one reason for wanting to be able to charge from the main bank.

Oops - you are quite right - again I was reading too fast.

I think the original idea has merit, even though it is not the most efficient. By using an inverter you can then run the AC through an extension cord out to the boat where the charger is, so you aren't schlepping an 80ah battery back and forth. I think that's the most practical way to handle it unless one needs the workout.

Perhaps something like the Noco GEN1 which is 10a and IP68 waterproof could live in the boat. I haven't checked the exact specs to make sure that it won't cook a gel, (despite their claim of being able to handle all chemistries - that's usually a compromise all around), but for non-critical agm charging, it might do ok. Plenty of options for chargers...

[SunEagle]

Oops - you are quite right - again I was reading too fast.

I think the original idea has merit, even though it is not the most efficient. By using an inverter you can then run the AC through an extension cord out to the boat where the charger is, so you aren't schlepping an 80ah battery back and forth. I think that's the most practical way to handle it unless one needs the workout.

Perhaps something like the Noco GEN1 which is 10a and IP68 waterproof could live in the boat. I haven't checked the exact specs to make sure that it won't cook a gel, (despite their claim of being able to handle all chemistries - that's usually a compromise all around), but for non-critical agm charging, it might do ok. Plenty of options for chargers...

The big issue I see with those Noco chargers is that they require AC power (which induces some losses) and they are expensive. I think a less expensive and easier way is to go with one of those RC battery chargers. Most already use a 12volt DC input (run from a car battery) and provide numerous battery chemistry charging profiles. Along with that they have an LED readout which provides some useful charging information and condition of the battery.

I use a Revolectrix Multi 4 which is only 4amp but they make higher amp chargers. Just another one to look at along with the ones that Sunking & jony101 provided.

[PNjunction]

I agree about the hobby chargers. Myself I use an iCharger 306B for various purposes both on AC and with regenerative feedback to an AGM source. Fun. Lots of menus, but one gets used to it, or create memory slots. Easy to read IR metering and so forth.

But for fishing, I think I'd just accept the losses, run an extension cord from the inverter out to the boat, with a waterproof marine type charger under the bench seat. Battery charging is fun, but sometimes fishing takes priority.

[Sunking]

Battery charging is fun,

Me thinks you need go outside more often. Try watching grass grow or paint drying.

[Jaakko]

One alternative, still a bit pricey, but get a Hobby Charger. By that I mean the ones used to charged Remote Control Airplanes and Cars with. They are made to charge any kind of battery and programable an most can charge a 1.2 to 36 volt battery from a 12 volt source. Something like this.

Thanks everybody for great answers! I clearly need to do more market research on hobby chargers. Previously I only found ones with relatively small current output, but it seems there should be more powerful ones with decent price tag.

There also seems to be a healthy DC-DC charger market particularly in Australia, and the prices of those (like Matson MA10DC) aren't that bad anymore. I'm just afraid with postage and duties it would effectively double by the time I'd get to Finland.

The battery I'm charging is mobile, since the boat is just a small rowing boat with no fixed place for the battery. However the 230VAC step would allow me to pull a longish extension cable all the way from the cabin to the dock (some 40 meters) without much loss. That way I wouldn't have to carry the battery back and forth.

Anyway, really appreciate all the ideas, and confirming that there isn't any silver bullet solution that I would've missed.

Jaakko