Battery Charger Sizing

[Piers]

From all I read, you take the battery amp hours, divide by 10, and that gives a recommended max charging rate.

I always thought amp hour rates were assuming a 10A discharge rate.

The batteries I have are rated at 255 amp hours, but at a 20A discharge rate. For calcualtion purposes, do I use an amp hour rate of 510?

 

[ccscott49]

I have a 75 amp three stage victron charger for my 550 ah battery bank, does me very well. In fact I have two, one spare.

 

[Simes]

Piers, Nice Boat!

work to 10% of the total battery bank or banks that you want to charge. I have found that this works OK. However if I had enough cash I would work to 20% of the battery bank.

If you have 2 x 225 amp engine start and 4 x 225 amp domestic then you would need a Charger of 135 amps (225x6=1350/10=135) I would go for a couple of chargers One supplying both banks and a second just supplying the Domestic bank (I would also have a switch allowing me to use both banks at once if required).

The Richardsons at Island Harbour are well able to help with this.

Hope that this helps

Simes

 

[Piers]

Hi Simes,

What I'm looking at is a charger with 3 outputs. I'd use one for the bank of 2, and another for the bank of 4.

Each batt is labelled as 255Ahr but at 20A discharge rate. So the qn is whether each batt is actually 255 or 510Ahrs as far as calculating the charger size.

 

[misterg]

255

 

[ccscott49]

If you are looking for a 3 output charger, I would go for either the 100 amp, mastervolt or victron. The battery capacity is 255 ah.

 

[Piers]

Thanks to all above. A 100A charger per bank it is.

 

[Anonymous]

Careful. Add your normal service load to the charger size. Do you have a generator? If so, even more important. Suppose you want to put back 200Ah into a 500Ah battery from the generator. Suppose you are running the watermaker (20A) fridge, lights, telly, PC, video, and so on running you up to, say, 30A standing load. Then you want 30A plus whatever you want to put back into the batteries.

Englander's numbers are nearly the same as mine (mine are 80A and 560Ah) but Englander is a 24V system so twice the power.

We work nicely with a Mastervolt 80A charger and 560Ah plus 180W solar both in harbour and at anchor in the Med, with a 6kVA generator.

 

[William_H]

The charger current ratings so far suggested are based on the idea that you want to take your boat to a jetty plug power in and recharge discharged batteries in the fastest time. So you can return the boat home.

If in fact you have 240v power available all the time then a far smaller charger will be fine. If you visit the boat once per week then you have 150 odd hours to recharge the batteries if you have discharged them over the previous weekend.
If however you have not discharged the batteries then you only need to replenish what you have used.

The most sensible way to rate your charger is to consider your max use when living on the boat at the marina. A charger which will handle all your steady loads without discharging the battery will almost certainly be able to replenish a battery after some use in a reasonable time ie several days.

So bottom line is a charger of modest capability will do nicely if you are have 240V power available 24/7 and are willing to leave the charger connected and charging while you are away. ie 10amp or 20 amp charger will probably suffice. olewill

 

[jfm]

Piers, there is a risk of mixed up maths here. You gotta be clear whether, when adding up your battery bank, you are allowing for the 24v. Say you have 4x255AH batteries in your domestic bank, at 24v. That is not 1020Ah of capacity as Simes suggested. It would be 1020Ah capacity AT 12VOLTS, but it's 510Ah capacity at 24v. If you want to use the 10% rule than you need a charger with 51Amps at 24v. Personally I wouldn't go anywhere that low and would install 75A - 100, but not becuase the 10% rule is flawed, rahter becuase as Lemain said you need to allow for the boat's overhead when it has a 230v mains source (dock or genset). If you use the 10% rule and add 25A of overhead, you need a 75A charger. That is exactly what my boat has: 4x255Ah batteries (so thats 510AH total capaity at 24v) and a 75A mastervolt charger

To answer your specific question, each batt is actually 255 as far as calculating the charger size, butt more precisely the domestic bank of 4 is actually TWO (not 4) 24v batteries of 255AH each, so your total domestics is 510AH at 24v, hence 51Amps under the 10% rule

Now to the separate 2x 255Ah starter batteries that are isolated from the domestic load. you do not need a 10% rule for these as it is implausible they would ever be heavily dicharged and need recharging fast. What they need is merely a trickle charge to keep them fresh for periods when the boat is standing. One of the little mastervolt intelligent trickle chargers is ideal.

So on your boat I'd have 1x 100A or 75A charger for the domestics, plus a separate trickle charger for the engine start.

 

[Piers]

OK - this has been so useful, let alone money saving! I have revised my requirements and am looking at Victron and Mastervolt options. Thanks for all the help.

 

[jfm]

FWIW, both Victron and Mastervolt are very good technically but I think Mastervolt offer outstnading after sales support. A few years ago I had a suspected problem on my then charger. I called Mastervolt and they sent me a new one FoC, from Amsterdam to France. They didn't want the busted one back first. Then it turned out I had typo-ed the email and asked them for the wrong model. My fault. I called to apologise and they just sent me another, the correct one, Amsterdam to France again. No hurry to get the old ones back. Top service, imho

 

[Piers]

I suspect Mastervolt uses a different technology from Victron, since Mastervolt's chargers appear to weigh so much less. They also look so good!