Sterling Battery Charger - temporarily adding a 4th battery?

[Mister E]

Just make sure you have the polarity correct on the new battery.
Then fit a fuse to match the charger, check that the cable from the charger to all batteries will handle the extra current.

 

[AndrewPalmer]

Sorry, I should add that the fuse was inline with the cables connecting the battery. Not shown in the picture, but I had another pair of cables with an Andersen plug and a 10A fuse on the red or black, can't remember which. The output of the charger has one red and three blacks (or the other way around). We connected to the one, and to one of the other three. If current flowing from the other already charged battery is an issue, will a 20A fuse do the trick, or will I need more?

 

[PaulRainbow]

Sorry, I should add that the fuse was inline with the cables connecting the battery. Not shown in the picture, but I had another pair of cables with an Andersen plug and a 10A fuse on the red or black, can't remember which. The output of the charger has one red and three blacks (or the other way around). We connected to the one, and to one of the other three. If current flowing from the other already charged battery is an issue, will a 20A fuse do the trick, or will I need more?

The charger should have one black (negative) and three reds (positives). Connect to the negative and one positive, with a 30a fuse in the positive.

The 30a fuse (rather than 20a) is to allow for surge and is only there to protect the wiring between the charger and the battery.

 

[Alex_Blackwood]

The charger should have one black (negative) and three reds (positives). Connect to the negative and one positive, with a 30a fuse in the positive.

The 30a fuse (rather than 20a) is to allow for surge and is only there to protect the wiring between the charger and the battery.

Just one question. Would you expect a 10-20% discharged 20 ah battery to take that much current? You may get an initial surge but it would be momentary.

 

[PaulRainbow]

Just one question. Would you expect a 10-20% discharged 20 ah battery to take that much current? You may get an initial surge but it would be momentary.

Wouldn't expect it to, but we don't know how depleted it will be, especially if it's being used with a trolling motor. The fuse is only there to protect the wiring, so no point taking chances, IMO.

 

[AndrewPalmer]

Just one question. Would you expect a 10-20% discharged 20 ah battery to take that much current? You may get an initial surge but it would be momentary.

The fuse blew immediately when we connected the battery. I'm sure it was not heavily discharged. Your point does lean into my question though really. If we assume that the existing battery on the charger is fully charged when I plug in the partially outboard battery, is that going to confuse the fancy charger, perhaps causing it to sent a spike?

 

[PaulRainbow]

The fuse blew immediately when we connected the battery. I'm sure it was not heavily discharged. Your point does lean into my question though really. If we assume that the existing battery on the charger is fully charged when I plug in the partially outboard battery, is that going to confuse the fancy charger, perhaps causing it to sent a spike?

No, i've explained this more than once.

 

[Boater Sam]

The fuse blew immediately when we connected the battery. I'm sure it was not heavily discharged. Your point does lean into my question though really. If we assume that the existing battery on the charger is fully charged when I plug in the partially outboard battery, is that going to confuse the fancy charger, perhaps causing it to sent a spike?

I would doubt it. The Sterling won't care how big a battery bank it is connected to, your extra battery is just like it making a bigger battery.
I suspect that you connected the battery back to front or that it is dead short duff.

 

[B27]

The fuse blew immediately when we connected the battery. I'm sure it was not heavily discharged. Your point does lean into my question though really. If we assume that the existing battery on the charger is fully charged when I plug in the partially outboard battery, is that going to confuse the fancy charger, perhaps causing it to sent a spike?

Not a 'spike' of voltage but a spike of current.
Your 90% charged battery is resting at say 12.6V
The charger will want to raise that to 14.4V or so.
It takes a certain amount of charge to do that, so for some seconds you have a 20A charger going through a 10A fuse.

If you connected the battery while the charger was switched on, you'd probably be connecting a capacitor on the output of the charger to the capacitance of the battery, so more than 20A could flow initially. If the charger had only been switched off for a few seconds, the capacitor in it may still have been charged.

My battery charger starts off with a few seconds of low current limit to minimise this kind of problem.

A little battery like that is probably outside the scope of what the charger designer was asked to produce.

 

[AndrewPalmer]

Thanks everybody, I am going to give it a go with a 30A fuse. I will wire the Andresen +ve to the single +ve (with the fuse inline) and the -ve to one of the three -ves. The charger will of course be off when I do that. To bel clear, I am not intending to disconnect the 'boat battery' -ve, but to wire my new Andersen OB charging plug alongside it (think that is called in parallel!).
I hope it does not really matter whether or not the charger is on when I connect up the OB battery, but will try with it off initially! Then whenever I need to top up the OB battery, I will not be rewiring again, but simply connecting it to the Andersen socket hanging out of the bottom of the charger.

The only other thing I think I have not mentioned is that the boat batteries are wet acid, whereas the OB batter is a sealed gel type. I hope that does not cause a problem.

 

[B27]

I think that battery is on the small side, both for a 20A charger and for a trolling motor.
I can't immediately see a Lucas recommendation for charge current limit, but other small AGMs typically want about 30% of the Ah rating in amps, maximum. Your looking at three times that. Discharging at 40A for a small-ish trolling motor is equally a bit edgy. Normal advice for trolling motors is a battery big enough for over an hour's run time, to avoid thermal problems, even if you're only running for a few minutes at a time.
I've seen the aftermath of an exploded lead acid battery, it's messy.

 

[PaulRainbow]

Thanks everybody, I am going to give it a go with a 30A fuse. I will wire the Andresen +ve to the single +ve (with the fuse inline) and the -ve to one of the three -ves.

This is wrong, perhaps i didn't explain it well enough. The charger has a single negative (Black) and three positives (Red). The negative of the trolling motor battery goes to the common negative of the boat. The positive goes to the positive of one of your existing batteries, not the charger. Your 30 amp fuse needs to be in the positive cable, but as close to the boat battery as possible. There should technically be a second fuse as close to the trolling motor battery as possible, but that needs to be rated for the current draw of the motor, providing the cables are big enough.

The charger will of course be off when I do that. To bel clear, I am not intending to disconnect the 'boat battery' -ve, but to wire my new Andersen OB charging plug alongside it (think that is called in parallel!).
I hope it does not really matter whether or not the charger is on when I connect up the OB battery, but will try with it off initially! Then whenever I need to top up the OB battery, I will not be rewiring again, but simply connecting it to the Andersen socket hanging out of the bottom of the charger.

It would be good practice to fit an isolator switch between the boat batteries and the Andersen connector, plugging in with the switch off, then turning it on.

The only other thing I think I have not mentioned is that the boat batteries are wet acid, whereas the OB batter is a sealed gel type. I hope that does not cause a problem.

Unlikely to be an issue.

 

[AndrewPalmer]

I think that battery is on the small side, both for a 20A charger and for a trolling motor.
I can't immediately see a Lucas recommendation for charge current limit, but other small AGMs typically want about 30% of the Ah rating in amps, maximum. Your looking at three times that. Discharging at 40A for a small-ish trolling motor is equally a bit edgy. Normal advice for trolling motors is a battery big enough for over an hour's run time, to avoid thermal problems, even if you're only running for a few minutes at a time.
I've seen the aftermath of an exploded lead acid battery, it's messy.

Thank you. I bought the small 22ah battery because of the weight, indeed I think the handbook recommends 65 or 85. I never thought that this might be connected to thermal issues, just range. I did check the battery specs to see that it looked like it could cope with the 50A discharge, but I assumed that the curve meant that the discharge would just tail off rather than cause thermal issues. I will monitor it.
I bought a couple of second hand 7ah batteries at a boat jumble at the weekend to overcome my range anxiety, perhaps better not pursue that plan.

 

[AndrewPalmer]

This is wrong, perhaps i didn't explain it well enough. The charger has a single negative (Black) and three positives (Red). The negative of the trolling motor battery goes to the common negative of the boat. The positive goes to the positive of one of your existing batteries, not the charger. Your 30 amp fuse needs to be in the positive cable, but as close to the boat battery as possible. There should technically be a second fuse as close to the trolling motor battery as possible, but that needs to be rated for the current draw of the motor, providing the cables are big enough.

It would be good practice to fit an isolator switch between the boat batteries and the Andersen connector, plugging in with the switch off, then turning it on.

Unlikely to be an issue.

Thank you Paul, If the +ves and the -ves are the other way around then that's fine, I will not get that wrong in practice. Connecting the +ve to the battery terminal would be more difficult though, as they are a distance from the charger, and I am not sure that another cable will fit through the cable runs. I did not imagine that to be any different to connecting to the terminal on the charger, which I assumed was directly connected to the battery itself?

 

[Alex_Blackwood]

I do wish people would read the thread and not just spout off with dubious theories.
1.Andrew has stated that he has a 20 Ah battery, brand new.
2. He has charged it at home on another charger and it is perfectly OK. (we don't know if that was before or after trying the boat charger?)
3. It is, he has stated, 80-90% fully charged.
4. The current capacity (20amp) of the charger is irrelevant as it is more than a battery of that size would be expected to demand. Even if pretty much fully discharged. I am surprised that a battery of that capacity would draw enough current ( 30amp ish.) to instantaneously blow a 10 amp fuse.
5. What the battery is used for is also irrelevant.
6. What we don't know is what he did with the connections! He does seem a bit vague on his +Ve. & -Ve.
7. Paul has given his usual concise and detailed instructions on how to connect the battery. I would however suggest that there should be no difference between connecting to a battery or connecting to the battery terminals of the charger. Assuming it is of course, done correctly. However do as instructed!
8. We must now, I think, wait until Andrew has carried out Pauls instructions and see the outcome.

Edit:-
Another consideration is the fact that if, when he connected the extra battery initially, if it was correctly connected it would be in parallel with the boat battery he had connected across. They would have tried to equalise, so why did the fuse not blow then if the 20ah battery was demanding such a high rate of charge.

 

[PaulRainbow]

I do wish people would read the thread and not just spout off with dubious theories.

That would be nice.

7. Paul has given his usual concise and detailed instructions on how to connect the battery. I would however suggest that there should be no difference between connecting to a battery or connecting to the battery terminals of the charger. Assuming it is of course, done correctly. However do as instructed!
8. We must now, I think, wait until Andrew has carried out Pauls instructions and see the outcome.

There is technically no difference, but i was thinking that the terminals, might not be big enough. But then i remembered, the Sterling charger has fairly long studs, so should be ok.

 

[PaulRainbow]

Thank you Paul, If the +ves and the -ves are the other way around then that's fine, I will not get that wrong in practice. Connecting the +ve to the battery terminal would be more difficult though, as they are a distance from the charger, and I am not sure that another cable will fit through the cable runs. I did not imagine that to be any different to connecting to the terminal on the charger, which I assumed was directly connected to the battery itself?

As per #26 I was thinking that the terminals might not be big enough. But then i remembered, the Sterling charger has fairly long studs, so should be ok.

I would still fit an isolator before the Andersen connector so you don't connect when "live".

 

[AndrewPalmer]

Thank you everyone! I have now rewired it using a 30A fuse, and it works perfectly as an external charger to plug in my outboard battery. No more need to carry a separate 12V battery charger on board!