We installed our NASA Marine BM-2 battery monitor in July of 2014 to help us solve the issues we were having with our Lifeline AGM batteries. So far, it’s solidly in the Yay category! Here are some details (Warning: technical material ahead)…
When we cruised ten years ago, we had a battery monitor that kept track of our battery state with incredible precision. We “upgraded” our electronics and the replacement has never known what’s up and what’s down where our batteries are concerned.
The problem our new Blue Sky IPN remote had is that when we’re out cruising, we seldom charge the batteries enough for the unit to reset itself to zero amp-hours. Never doing this, it doesn’t get a chance to readjust it’s opinion of the battery efficiency – always thinking it’s 99% (I wish!). I tried reinstalling old-faithful but for reasons that are a mystery to me, it no longer keeps track very well. Maybe it’s that we now have AGM batteries.
Enough is enough. After some research, I found the NASA Marine BM-1 and BM-2 battery monitors online and read all about them. I decided to order one when I read that it used two methods to keep track of battery state; amp-hour integration and Peukert’s equation and because of this, only needed to “observe” the batteries for a short time before making an educated guess at their state of charge.
Let me explain amp-hour integration if you’re interested. Most battery monitors use a current shunt which is basically a bunch of brass fins or tubes with a known resistance to the flow of electricity. When electricity flows over the shunt, there is a very slight voltage drop. By measuring that voltage drop, it’s easy to calculate amps (current) passing through the shunt. Taken over time, the result is amp-hours (One amp passing through the shunt for one hour means you’ve used 1 amp-hour.)
With this approach, you need some way to reset the amp-hours on the meter. There are all kinds of schemes for that reset. The meter can look to see that the battery voltage is high enough and the current coming in small enough that it can assume charging is finished and the batteries are full. Problem: If you are using solar panels to charge and the sun goes behind a cloud, the reason the current is small is that the solar panels can’t put out any more power and the battery may not be charged when the meter thinks it is. During the cruising season, we never get our batteries charged enough for a true reset.
You’d think that all battery monitors would have manually set or reset function, but no. The NASA Marine monitor does with an easy way to set the level to zero. By doing that, when our batteries are full enough (or as full as you’ll ever get them) we can set our zero level and use it just like a manual counter (using amp-hour integration to count amp-hours in and out).
More explanation: It doesn’t matter if the zero level really represents a completely charged battery. If you charge your battery to 80% and discharge it to 50% and your battery bank is 400 amp-hours, then zero it at what you think is 80% and turn on the engine when it hits 120 amp-hours discharged. (80% – 50% = 30% of 400 amp-hours or 120 amp-hours.) Done. Simple.
But there’s more to the BM-1 and BM-2… It also uses some clever math stuff (Peukert’s equation) to take a guess at the battery state using amps coming out or going in and voltage. It’s guess seems pretty good, not perfect, but pretty good. I think it might have a bit of an issue with our AGM batteries, but it’s guess has been more than usable for keeping our batteries up and knowing when to charge. (See update below.)
Because of the math its little brain does, it never really needs to be reset to zero. On all three of the display screens, it shows a 0 – 100% scale with your current battery state clearly shown.
We’ve used this for about a month now (having ordered it on Amazon and shipping it to Tonga) and I’m very happy. The only two negatives are that it’s pretty large and I don’t like the dark corners on the screen when the light is on (though that’s explained away in the manual). This is not something you’ll see in a West Marine and it’s really worth looking into if you are having any trouble with battery monitoring. Here’s a link to their website.
By the way, the difference between the BM-1 and the BM-2 is only that the 1 has a 100 amp maximum current in our out and the 2 a 200 amp maximum. We got the 2.
Update:
It’s now the end of August and we’ve been using the monitor for over a month. I’m still very impressed, more so than when I wrote the above. Here’s why…
It let us see what sad shape our Lifeline AGM batteries were in. On the monitor, you can set your battery bank amp-hour capacity. By incrementally moving it down from what was supposed to be the capacity of our battery bank (440 amp-hours) to about 150 amp-hours, the monitor tracked our battery state with much greater precision. We’d lost more than half of our battery capacity!
We just finished equalizing our batteries as per Lifeline’s instructions and it looks like we’ve restored the batteries’ capacity to a little better than 400 amp-hours. The battery monitor is tracking incredibly well now. I would highly recommend the NASA Marine battery monitor!
Another Update – a month later…
After just a few weeks, our AGM batteries are right back where they were: shot! They were like new for a week or so, and then the capacity started to drop. We replaced them in Suva with tried and true, flooded cell, Trojan T-105s like we’d had before this failed Lifeline AGM experiment. Flooded cell batteries picky about being charged to 100% every few charge cycles.
Another Update – a year later…
When in Australia, we had a bimini built and put a Sunpower E-20, 327 watt solar panel on top of it. We also replaced our Blue Sky (boo) solar controller with two Victron Energy 100/30 MPPT controllers. The result is that we now have enough solar charging capacity (487 watts) that, even on cloudy days, we get to 100% by about 10 AM! We probably could have kept our AGM batteries. With this charging arrangement, I doubt they would have gone bad in the first place. Big YAY for both the Sunpower panel and the Victron controllers!