Sizing Battery Cables

[capricorn]

Hi,

I'm adding a domestic battery bank to my boat this winter and as a result of moving the batteries and isolator swich around need to replace/add new battery cable.

Could someone advise on the correct size battery cable to use ?

I have a Yamnar 2GM20 with the standard 70a alternator (as far as I know). The new battery setup will have an 85ah starter battery and (probably) 2 x 105ah domestic bank.

Due to the less than ideal location of the domestic bank some of the cable runs will be quite long - maybe 8 metres round trip from battery to starter. I was thinking of using 35 sqmm flexible multistrand cable which is rated for 220 amps, does this sound about right ?

I know how to calculate the volts drop for a given cable/current but don't know how many amps the starter motor draws, can someone give me a ball park figure ?

 

[PCUK]

Is that 8 metres from battery to starter for positive and negative i.e. 4 metres actual run or 8 metres for both positive and negative, giving a total run of 16 metres?

 

[capricorn]

No 8 metres round trip, ie 4 metres from battery to starter.

 

[abraxus]

You may want to go larger than that. Here's a wire sizing chart in any case:

 

[CSFenwick]

I had a similar dilemma earlier this year and couldn’t get a straight answer from anywhere on what the max current draw to the starter motor is.

The boat came from new (Moody) with AWG2 (35 sqmm) and that had worked fine so where I needed to make changes (battery bank mod’s) I used that again. I should add that the changes I made are only around the battery bank which is in the aft cabin, the original AWG2 cables still carry the load to the switches. The run on my boat sounds slightly longer than yours.

The table posted by Abraxus is certainly the starting point for these calc’s but because it shows maximum constant current capacity it implies incredibly think wire for starter motors – far thicker than I’ve seen on any boat. The current draw is clearly very high when the starter motor spins but it only lasts a second or so hence the cable doesn’t get hot – or even warm on board my boat.

I’d be keen to get other people’s take on this and to know what’s fitted on their boats – particularly what was supplied from new.

 

[Graham_Wright]

[ QUOTE ]
incredibly think wire

[/ QUOTE ]????!!!???

Are we hedging our bets? /forums/images/graemlins/smile.gif

 

[DavidMcMullan]

From a Yanmar 2GM20 brochure, I understand that the starter is rated as 12VDC 1.0KW.
For the starter circuit it states for total +ve cable length, if <2.5m 20mm crossection area; if <5m 40mm CSA.
These numbers may allow you to scale up.

 

[capricorn]

I posted the same question elsewhere (Yanmarhelp.com) and a chap there has advised to to specify the cables such that I will still be able to provide 10v to the starter whilst it's delivering the maximum stall current of (apparently) 280 amps.

I haven't been able to find a figure for the internal resistance of the leisure type batteries that I intend to use but I know that the internal resistance of a dedicated gell start battery is of the order of 8 milli ohms. My quick calculations show that the battery's internal resistance alone will cost 2.25V for that current. On that basis I should aim to lose no more than another volt to the cables which by my calculations means I need to specify at least 45 sqmm cable assuming a 10m round trip.

This is annoying since I can source 35sqmm welding cable quite cheaply but 50sqmm cable comes in at around 3 times the price. That said what price if I can't start the engine to get me off a lee shore one day because I scrimped 40 quid on the battery cables ?

Any thoughts ?

 

[pvb]

Some thoughts...

Given an adequately-sized (and charged) battery, the 35 sq mm cable you're proposing should be fine. The cable run seems very long though, you first mentioned 8 metres and it's now up to 10 metres - is it really impossible to locate a starting battery a bit closer to the engine? Also, I'm not clear about whether you're planning to use a gel starting battery (which you mention). Gel batteries are very good but they generally have a lower Cold Cranking rating than similar-sized wet batteries. Gel batteries also need a carefully controlled charging regime, so if you mix gel and wet batteries you'll create charging difficulties. Your ideal solution would probably be to use maintenance-free batteries and try to get the start battery closer to the engine.

 

[duncan]

Re: Some thoughts...

solid response - split the batteries and have the long run to the domestics from the charging circuits rather than the load circuits to the starter.

having then cut down the lengths required for the load leave the welding cable alone and do the job properly.

 

[capricorn]

Re: Some thoughts...

Yep,

Think you've hit the nail on the head. The solution I've been toying with involves entending the cable runs of the existing single battery (which will become the dedicated start battery) in order to be able to fit a rotary selector switch in a convenient location. Agree that's probably not particularly wise. I'll think again so as not to have to extend the cables from the current battery locatiion.

Thanks

Edit - Pvb, thanks also. My reference to the gel battery was since that was the only battery for which I could find an internal resistance figure for. My intention is to use a single 85ah lead acid leisure battery for engine starting - that's what's started the engine and run the domestic supply for the last 14 years, and 2 x 105 ah lead acid leisure batts for the domestic bank

 

[pvb]

Rotary selector...aarghhh!

Hate rotary selector switches - too much room for error. Much much better solution is 2 simple on/off isolator switches; one in the starter circuit, one in the domestic circuit. You might also find you need much less heavy-duty wire, depending on where you put the start isolator. Alternator output should go direct to the starter battery, then use a VSR to charge the domestic bank. Foolproof and failsafe.

 

[capricorn]

Re: Rotary selector...aarghhh!

The current isolator setup has an isolator switch for both the positive and negative sides of the existing single battery. Do you think this is overkill and it's only necessary to isolate the + side of each battery ? If so then I've got a pretty simple job to use the existing to isolators to isolate the starting and domestic batterries in the manner you describe. What do you think ?

 

[pvb]

Yes...

Yes, it's fine just to isolate the positive cables. Using your existing isolators would be a cost-effective result too!

While you're doing the wiring, it would be sensible to add a Mega Fuse to each bank, close to the battery.