Wire size?

[GeorgeLlewellin]

I want to fit a 240v battery charger to the boat, but the nearest I can fit it to the battery banks is about 5m away. The charger can charge two 12 volt battery banks separately at up to 25amps, I thought to run two separate twin core tinned wires from charger to batteries, but what size do they have to be?

Also is there a simple formula that I can use to calculate this for myself in future, assuming a maximum of 3% voltage drop.
Thank you George

 

[VicS]

I want to fit a 240v battery charger to the boat, but the nearest I can fit it to the battery banks is about 5m away. The charger can charge two 12 volt battery banks separately at up to 25amps, I thought to run two separate twin core tinned wires from charger to batteries, but what size do they have to be?

Also is there a simple formula that I can use to calculate this for myself in future, assuming a maximum of 3% voltage drop.
Thank you George

Loads of calculators on line. I like this one https://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

For 25 amps and 10 m ( 5m positive + 5m negative) wire length it gives between AWG 6 and AWG 4

If arithmetic is your thing then you can calcuate the wire size needed using the data in this table: https://www.engineeringtoolbox.com/wire-gauges-d_419.html

If you use any other online calculator be sure you know if its based on the distance ( eg in your case, 5m) or on the total wire length ( 10m). They don't always say!

 

[PaulRainbow]

No idea why Vics always quotes AWG sizes for cable. 15mm and 20mm are rather oddball sizes, standard battery cable sizes would be 16mm and 25mm.

16mm would be the minimum. I doubt the need for two negative cables, what charger is it ?

 

[KenMcCulloch]

I’ In the middle of improving the idiosyncratic electrical installation of my very recently acquired boat, and have found the Blue Sea Systems phone app (free download) very useful as guidance on wire sizes and fuse or breaker sizing.

 

[BabaYaga]

Another way of estimating this could be to look at what reputable manufacturers supply. One example:
CTEK M300 is a 25A single output charger. It comes with a 4m charging cable, so 8m round trip. Wire size is 5.3mm square.
My conclusion is that you should not calculate this by traditional voltage drop methods, likely because the charger itself compensates for voltage drop.

 

[PaulRainbow]

Another way of estimating this could be to look at what reputable manufacturers supply. One example:
CTEK M300 is a 25A single output charger. It comes with a 4m charging cable, so 8m round trip. Wire size is 5.3mm square.

In contrast, Sterling quote 25mm cable for a 5m run of cable with their 20a chargers or 50mm cable for the 30a charger.

My conclusion is that you should not calculate this by traditional voltage drop methods, likely because the charger itself compensates for voltage drop.

Unless the charger has battery sensing, that's not likely.

 

[BabaYaga]

Unless the charger has battery sensing, that's not likely.

If a charger does not sense battery voltage, what voltage does it sense?

 

[VicS]

Another way of estimating this could be to look at what reputable manufacturers supply. One example:
CTEK M300 is a 25A single output charger. It comes with a 4m charging cable, so 8m round trip. Wire size is 5.3mm square.
My conclusion is that you should not calculate this by traditional voltage drop methods, likely because the charger itself compensates for voltage drop.

You are quite right of course. Not because the charger is sensing the volts at the battery and compensating for the volts drop in the cables but because during the bulk charging stage and the early part of the absorption stage the volts drop is not so important.

It becomes important in the later stages when one wants to apply the full controlled voltage ( eg 14.4 volts ) to the battery. By then of course the current will have fallen to possibly between 1 and 2 amps, depending on battery size. OTOH a 3% drop may not be acceptable so perhaps one should be looking at a 1% drop or less at this current .

The AWG 10 cables fitted to the Ctek M300 would result in 0.054 volts drop at 2 amps and 0.14 volts drop ( 1% of 14 volts) at 5 amps.

 

[VicS]

It does not sense any external voltage..... It outputs the specified voltage for the battery type selected.

 

[BabaYaga]

Not because the charger is sensing the volts at the battery and compensating for the volts drop in the cables but because during the bulk charging stage and the early part of the absorption stage the volts drop is not so important.

Agreed.

 

[PaulRainbow]

If a charger does not sense battery voltage, what voltage does it sense?

If a charger were to be able to compensate for voltage drop between the charger and the battery it would need to be able to sense the voltage at the battery independently. I've not seen a "marine" charger that does this. It would be a pointless feature, much better for the installer to use the correct cable.

 

[VicS]

If a charger were to be able to compensate for voltage drop between the charger and the battery it would need to be able to sense the voltage at the battery independently. I've not seen a "marine" charger that does this. It would be a pointless feature, much better for the installer to use the correct cable.

Correct and for the reasons explained in #8 they do not have to be as heavy as originally calculated.

 

[pvb]

Correct and for the reasons explained in #8 they do not have to be as heavy as originally calculated.

Which makes me wonder why Mr Sterling specifies such big cable.

 

[lpdsn]

I want to fit a 240v battery charger to the boat, but the nearest I can fit it to the battery banks is about 5m away. The charger can charge two 12 volt battery banks separately at up to 25amps, I thought to run two separate twin core tinned wires from charger to batteries, but what size do they have to be?

How big are the batteries?

To cut a very long story short, when I bought my boat I found the previous owner had fitted a new mains charger, but he hadn't connected up the starter battery. It was a tight run of about the same as yours so difficult to run thick cables, so I just used 2 X 2.5mm2 for the starter battery positive. The negative was as built so about 40mm2.

As the starter battery is just 55Ah and is used for nothing else I don't see the cable size as a problem. Even if it got drained a bit as a result of starting problems it would eventually be brought back up to voltage. Normally though there's not much charge to put back into it. The domestic bank is another story, but thick cables for that were already there.

 

[GeorgeLlewellin]

Thanks everyone for such a quick and informative replies, I will now look at the links and study it in details.

I will I hope, before I finish, create a spreadsheet into which I can enter all the relevant information and get a result of wire size required.
Regards, George

 

[PaulRainbow]

Which makes me wonder why Mr Sterling specifies such big cable.

The Sterling chargers (pro charge ultra etc) will operate as a power supply. My 30a charger will churn 30 amps out without a battery connected. In the case of the little Ctek quoted earlier, if that was running as a power pack @ 25a it would be suffering close to 6% voltage loss.

I still think Sterling are being OTT with their sizing though, even if you do the calcs for a constant max output of the chargers.

 

[BabaYaga]

The Sterling chargers (pro charge ultra etc) will operate as a power supply. My 30a charger will churn 30 amps out without a battery connected. In the case of the little Ctek quoted earlier, if that was running as a power pack @ 25a it would be suffering close to 6% voltage loss.

I still think Sterling are being OTT with their sizing though, even if you do the calcs for a constant max output of the chargers.

The CTEK M300 does not have a supply mode, but another CTEK 25A model, the MXS25000, does.
When in supply mode this puts out 25A @ 13,6V.
Provided it is using a similar charging cable as the M300 (information not found) it would still supply at around 12,8V.

 

[lw395]

3% loss when your charger is regulating at 14.4V is 0.432V
So your battery will only see 13.97V

If you get yourself a variable power supply and a battery, you can see that the current into say a 60% battery is a whole lot less at 14V than 14.4V
So what will actually happen is the charger will be at 14.4, the current will be something like halved and the battery will be at say 14.2V.

Any cable drop slows charging significantly once you are in the constant voltage phase.
Does this matter?
Very often no it does not as many boats have long periods on shore power to get fully charged.
But a liveaboard who wants to get best value out of running the generator should not be tolerating this kind of voltage drop.

PS, if you actually do the experiment, you need to let the battery settle, draw a bit of load current between each reading then look at the charge current after a couple of minutes of charging.

 

[VicS]

3% loss when your charger is regulating at 14.4V is 0.432V
So your battery will only see 13.97V

If you get yourself a variable power supply and a battery, you can see that the current into say a 60% battery is a whole lot less at 14V than 14.4V
So what will actually happen is the charger will be at 14.4, the current will be something like halved and the battery will be at say 14.2V.

Any cable drop slows charging significantly once you are in the constant voltage phase.
Does this matter?
Very often no it does not as many boats have long periods on shore power to get fully charged.
But a liveaboard who wants to get best value out of running the generator should not be tolerating this kind of voltage drop.

PS, if you actually do the experiment, you need to let the battery settle, draw a bit of load current between each reading then look at the charge current after a couple of minutes of charging.

Except that as the battery charges the current will reduce. You may have 3% volts drop at the max charger output current but as this reduces so will the volts drop . By the time you have reached say 5 amps it will only be 1/5th of what it was at 25 amps. Approximately 0.086 volts. At lower currents it will be even less.
This is why the Ctek chargers will work even though the battery connecting cables are relatively small compared with what you calculate to be necessary if you base your wire size calculation on the max charger output.

The Ctek chargers are basically compact portable chargers for which having several metres of AWG 4 or AWG 6 as battery connections would be a complete nonsense. The cables would be bigger than the charger.

I thought I had already explained this.

 

[Neeves]

The question of cable size is fairly common, whether for charging, in this case, or consuming (the common one are cables to the windlass - which is usually a long way from the batteries and source of charge, alternator).

Excuse my complete ignorance - but given that there is a recommended size, and I understand why one should not use smaller cables, but is there anything wrong with using cables that are too big, other than its nonsense, they cost more, they might not fit in tight spaces and they are heavier? I have not looked at cable prices and maybe bigger ones are factorially higher, especially ones the terminal is crimped on - but anyway??

Jonathan