I used to have a Victron Multiplus in my previous vehicles but this is now overkill.  I always used the Victron BMV battery monitor and found it to be very accurate and a useful bit of kit.

In my new van I still wanted a battery monitor but decided to go with the Nasa Marine unit BM-1.

Having used this for a couple of weeks now I cannot recommend this bit of kit for the following reasons.

• There is no reset button to set the unit to 100%.  So quite often I have seen it showing 105% after being on charge.
• The % notification fluctuates wildly.  Now this may be because the unit is accurate but it makes it pretty unusable for me. (more below)
• The time left at current usage levels follows the actual usage too quickly.  So when my fridge goes on it drops to say 50Hours remaining and then quickly jumps back up to 199Hrs when it goes off 90 seconds later. This is just a niggle.

With the Victron,  you hold two buttons down to reset it back to fully charged.  This is handy as it gives the meter a known good state.  The Nasa Marine unit doesn’t have this feature.  I have just disconnected the unit to move it and after reconnecting with a full battery it is showing 0% and I have no way of getting it to 100%.  It is slowly creeping back up to 100% but it may take an hour or more.

When I am off hookup and need to know the % left this meter is only good to within 5%.  Say I am at 90% and the fridge switches on it drops to 85% and after the fridge goes off it slowly climbs back up to 90% again.  If I look at it at any time during this cycle I am given an inaccurate figure.  I am guessing this is because the Nasa unit measures accurately and immediately.  As we all know when you use a battery its actual capacity is reduced by a factor directly related to how fast you are drawing current.  However this fast update of the display makes it a pain to use.

The victron unit does all the same calculations as the Nasa unit but it appears to average out usage so the % meter doesn’t fluctuate wildly like this.  This may not be as accurate on a second to second basis but it is much more useful and practical for day to day usage.

The victron also gives you an AH left in battery reading which I always found very handy.

The downside to the Victron is the much smaller display.  However I see this as a bonus as it saves space…

The Nasa unit Costs around £100 and the Victron costs £130 and I would say go for the Victron.  The extra cost is well worth it.

[Update]

I just thought I had better mention.  I am highly reliant on 12v power for both my business and living so my criticisms may not apply to the normal motorhome user who just needs to keep a rough track of their usage.

An hour after I made this post the BM-1 is showing 104% which is patently not possible.  I am now really frustrated with the meter and I will be upgrading to the Victron BMV-600.  Anyone want a cheap Nasa BM-1?