Battery monitor

[DaveS]

I would doubt that this is a real issue unless your starter motor (and engine) is particularly big and you crank for a long time. Nasa's instruction leaflet quotes 102A as the highest continuous current it will measure and a 25% overload for a few seconds (assuming a 1.4kW starter) is unlikely to melt the shunt. (The shunt drops 50mV @ 100A, i.e. dissipating 5W; at 130A this dissipation would rise to about 8.5W. It will get warmer, but it shouldn't melt.)

There are, however, other very sound reasons for keeping starting and service battery functions entirely separate in normal circumstances...

 

[simon_sluggett]

Re: Overkill...

The goverment wants us all to have a similar device in our homes to make us realise how much power we use or waste.
So you'll be able to lie on the settee in the comfort of your own home and shout at the kids etc. etc. /forums/images/graemlins/laugh.gif

 

[Brayman]

Re: Overkill...

Perhaps all our homes should run off 12v then we would be more careful about how much power we use!

 

[gandy]

If you use the Nasa monitor, could you still use the battery for engine starting, with the current going through the shunt?

 

[roly_voya]

You could have problems if you just fit the battery monitor! In order to calculate the state of charge of the batteries the monitor measure the electricity draw out of the battery, thats no problem. When you recharge with the sw set to 1+2 you must make sure that only the charge going into the domestic bank is recorded so the shunt must be between the sw and the domestic battery. If you set it up to read the total charge from the alternator to both batteries, ie connect it before the sw, then you will confuse it because it will also be recording any charge going into the start battery as well but only reading the discharge from the house bank. This is not a major problem because you draw almost no charge from the start battery, a simple charge indicator or basic volt meter will show that the start battery is being charged is sufficient.

The main problem with 1+2 battery switches is that soner or later they either fail or get left switched to 1+2 and then you get a ded start battery. This wasn't to bad in the old days when engines had hand cranks but now can be an embarrisment. Diode splitters are a much better bet and quite cheap. The disadvantage is that they cause a voltage drop between the alternator and battery (but not the batt charger which has seperate outputs for each batt). The answer to this is to also fit a smart alt charger (as suggested further down) this not only gets over the volt drop problem but charges the battery in about 1/4 the time. If you put the unit where it can be seen it also has an 'idiot light' that tells you when the battery is charged. For a total of about £250 this will revolutionalise you electrics and probably be a better investment than a batty monitor although you could still fit one if funds alow but if it was me I would do the other changes first.

PS if you do fit a smart alt charger you will probably need to replace both the pos AND earth lines to the battery as the alternator will run at full output, probably around 50-70a as a pose to the likely 10-20a you will get from a standard alt regulator. So unless you want heat and light as well fit BIG cables

 

[pvb]

Some confusion here...

[ QUOTE ]
You could have problems if you just fit the battery monitor! In order to calculate the state of charge of the batteries the monitor measure the electricity draw out of the battery, thats no problem. When you recharge with the sw set to 1+2 you must make sure that only the charge going into the domestic bank is recorded so the shunt must be between the sw and the domestic battery. If you set it up to read the total charge from the alternator to both batteries, ie connect it before the sw, then you will confuse it because it will also be recording any charge going into the start battery as well but only reading the discharge from the house bank. This is not a major problem because you draw almost no charge from the start battery, a simple charge indicator or basic volt meter will show that the start battery is being charged is sufficient

[/ QUOTE ]I think it's likely that you haven't actually used a battery monitor. The convention in installing them is to put the shunt in the negative wire of the battery/bank to be monitored. This automatically ensures that the monitor only measures the current flowing into or out of that battery/bank.

 

[Chris_Robb]

Thanks for that - I was worried if I forgot to change over batteries, that I would cause problems. And yes I normally keep both functions separate, however - with 2 x 100AH on the engine bank, and 1 single 220 on the domestics, it would be better to move 1 of the engine batteries into the domestic bank - except the batteries are not identical - can you mix sizes in battery banks? Also the engine batteries are 5 years old - domestics are new. All electsol - which was probably an overpriced mistake. (particularly nasty people to deal with I have found)

 

[DaveS]

As I understand it - and there are others who know far more about battery chemistry than me - there is no problem in parallelling up different Ah sizes of battery provided they're all of the same type, i.e. flooded cell or whatever. The battery voltages have of course to be identical - which is obvious when comparing labels, but a bit more of a problem if a cell goes short circuit ...