nasa battery monitor

grafozz

grafozz

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i fitted a b m 1 and have 4 batteries, 2xbanks works fine , very useful, but its connected to one battery ,when both battery banks are switched to "all " is the reading given , a reading for all four batteries or just the one its connected to ? anyone fitted a b m 1 to two battery banks ? ?
 
I have mine on three battery banks, but they are all completely seperate. If the switch is in the "both/all" position, it reads the voltage of both banks of batteries, as they are now in paralell, electrically. In fact the seperate readings on the BM1 will be or should be identical. You need two shunts to connect it to two banks.
 
I asked nasa the same question and they sent me this useful FAQ.

The BM-1 was designed to monitor the boats service battery. This battery can be made up of any number of individual batteries up to a total capacity of 650 AMP hours. The unit measures voltage and current flowing in and out of the service battery and calculates the percentage of charge left in the battery and predicts the time to discharge or charge.

Unlike many monitors, this unit will not get ‘out of step’ with the battery. It was a novel system to keep track of the battery state of charge (SOC) and regularly re-synchronise itself.

The flexibility of the system has encouraged many users to make the unit do more than was intended. This is fine so long as certain rules are followed. Below are some of the set-ups we have heard of.

The voltage of the starter battery is measured by switching the positive supply to the BM-1 over to the starter battery. This is ok as long as it is not left in this condition for long periods. The BM-1 would see the voltage across the starter battery and the current flowing from the service battery and would be unable to correctly calculate the state of charge. Keep the period whilst reading the starter battery to no more than a couple of minutes. Do not
attempt this operation whilst there is a net charging current. Do it only when there is a net discharge.

A pair of service batteries have their negative terminals connected together and go through the shunt to the boats electrics. A switch in the positive live selects either one, two or both batteries. The output of the switch being connected to the BM-1 positive line. This is acceptable but you must understand what is happening. Suppose both
batteries are 100 AMP hour and the BM-1 is set for a 100 AH battery capacity. Then if one battery is selected the BM-1 will accurately track its state of charge. Swapping over to the second battery (only to be done when there is a net discharge current) the unit will quickly reset itself to reading the state of charge of the second battery. If both batteries are connected together you now have 200 AMP hour. This system will still provide a reasonable indication of the state of the pair of batteries, however it must be remembered that it will now take twice as long to charge or discharge than the BM-1 predicts.

Whilst the BM-1 is designed to be on all the time, the positive is corrected through a circuit breaker so goes off when the boat is left. This is ok but when you turn the supply back on you need to put a load on the battery (turn on some lighting say) wait until the percentage state of charge stabilises before commencing to charge the battery.
This should only take a minute or so.

The shunt supplied with the BM1 is rated at 100 Amps continuous. It is capable of considerably more for short periods. As long as the shunt is not overheated no damage will occur. Spare shunts are available direct from Nasa Marine.
 
Shame...

[ QUOTE ]
An intelligent and well written response from a manufacturer for once.

[/ QUOTE ] Shame they can't do the same with their website and their product manuals! /forums/images/graemlins/laugh.gif
 
Re: Shame...

Can't say I actively looked. My perception is that others are much more expensive. I wouldn't normally buy Nasa instruments, and in truth this one is a bit 'clunkey'. But it does exactly what it says on the tin, was a doddle to fit, seems very accurate and tells me exactly what I need to know; no more, no less.

Not bad for £100. Have I missed something?
 
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