Nasa BM1 shunt

[Georgio]

I have done a search for a definitive answer but can find one so here goes...

I'm looking to fit the BM1 battery monitor, I have two 120Ah batteries that are used for both domestic and engine start duties. I would quite like to leave the batt selector switch set to both (treating them as one bank) and monitor our usage. The basic premise is that I would reather run both batts down to 50-60% and then charge than run one very low (and possibly damaging it) before re-charging. Does that make sense?

If the worst happends I can start the engine by hand (there is a bit of a knack to it but easy-enough now).

However - my question. Is the 100A shunt goinh to be too small or potentially damaged by starting my 2GM20 through it??

I'm guessing it may get a bit warm for a few seconds but should be ok?

I realise the ideal solution it to have a seperate dedicated start battery but I don't want to go down that road just now.

 

[Clyde_Wanderer]

Georgio.
I have just been given a BM1 for xmas, and am in similar type of predicement, only I have two banks, service, and starting, obviousley it will be monitering the service bank, but I have emailed Nasa today with the question about connections, as it wont be to just one batt, but 2x110A parralled.
I sent them my wiring setup diagram too.
So it will be interesting to see if they get back to me and if so, what they say.
You should maybe check with them too, because if you damage your BM1 it wont be covered by warranty.
nasamarine@aol.com
Cheers,C_W

 

[halcyon]

In the past most starter motors ran at 200 - 300 amp, stall current can be 400 - 500 amp, now you can get 500 amp running, and 1200 amp stall.

A 2GM20, from my gen runs at 175 amp, and pulls 400 amp at stall, so you could have 300 amp at start up.

Brian

 

[galeus]

I'm not certain whether I'm any help,but I have the same (is it BMEA ?) battery monitor;for about 3 years and my boat is wired up conventially through 1 2 all,and I often start mine up with all 3 batteries,1 x 88ah & 2 x 85ah.All remains OK>

Richard

 

[galeus]

Sorry Georgio I've just seen the NASA bit.

Richard

 

[pvb]

If you ask Nasa, they'll tell you that it shouldn't be done that way. However, the high current drawn by the starter motor shouldn't damage the BM1 monitor (although it won't read the current accurately) and shouldn't damage the shunt. As starting the engine takes only a few secxonds, the overall inaccuracy in your monitor readings won't be too important.

 

[philip_stevens]

It is wired up the same as the battery monitor I have in our boat - though a different model.

As the shunt is on the negative side of the battery, and the 1/2/both/off switch is on the positive side, the general domestic load should be from the shunt, and the engine starting negative should be from the batteries common negative from the engine frame. This way, whichever battery you decide to use to start the engine, will not go through the shunt, and all domestic loads will go through the shunt.

I took a new 25mm auto type cable from the negative that was common to all 3 domestic and engine starting batteries, to the shunt. All domestic negatives come off the other terminal of the shunt. All domestic positives will therefore be taken from the domestic batteries positive or the main terminal on the 1/2/both/off switch that will be from your nominated domestic battery (via a fuse bank or separately fused). Otherwise take it from the common on the main 1/2/b/o switch - again, via fuses or circuit breakers.

Ideally, better than a 1/2/both/off switch, would be two isolating switches - one each for engine and domestic services, and maybe a third emergency interconnecting switch.