NASA Battery Monitor

[lumphammer]

I have been thinking of getting one of these, but would like to find out how best to connect it in the standard setup with two batteries and a 1-2-both switch.

I am looking at putting the shunt in the common negative return, so it will monitor the current drain of whatever batteries are connected.

Does this make sense? Is the shunt likely to be damaged by the engine starting current, which I guess could be more than 100A?

Any other thoughts on how best to set this up gratefully received.

 

[Twister_Ken]

ST has reviewed it and it seems as if it's one monitor, one battery (bank). Not poss to switch between two batts coz it 'learns' about the batt is connected too.

 

[pvb]

The starting battery is virtually fully charged all the time (starting takes only a tiny amount of power from the battery), so there's no point monitoring this battery. Fit the monitor so it measures the current into and out of the domestic battery (or batteries) - this is the information you'll find useful.

 

[alienzdive]

Is it also useful to monitor a fully charged starter battery rather than make assumptions?? Is that not what battery monitors are for??

 

[pvb]

No. Starting an engine uses maybe 1Ah, which will be replaced very quickly. Why would you want to monitor it? If it's only used for starting, how's it going to lose its charge?

 

[alan17]

In time all batteries will fail. I would want to monitor the engine start battery to be certain that I can start the engine. Thereafter with the engine running all other items will probably be OK. Also if domestics fail lt ls hardly a disaster but if the engine start fails when needed in an emergency a disaster could follow.

 

[rwakeham]

I think (from memory) the ST article said that it was specifically for monitoring the domestic battery.

 

[William_H]

Hello Alan A battery monitor will not help in predicting the failure of a battery.(except perhaps the voltage monitor) It is intended to monitor what goes in (amps) and what goes out as a more accurate measure of a battery's charge state.
The humble voltmeter would be a more usefull predictor of imminent battery failure if it is monitored for actual voltage while cranking the engine. The other predictor is cranking speed.

The batttery monitor (SFAIK) is normally used only on the domestic battery and that battery should be kept only for domestic service while the engine battery should be kept for the engine. If you swap the roles the measure of cumulative charge will be confused.

However this regime of dedication of batteries has a disadvantage. The domestic battery should be able to start the engine or at least assist when the engine battery dies. You need to check occasionally that the doemestic battery can do the job as it could fail in terms of engine start while appear OK for domestic service. This could leave you unknowingly with no engine start standby. olewill

 

[hubert]

I asked nasa the same question and they sent the following FAQ's.

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.