Cheap but Effective Battery Monitor

[slode]

While volt meters are good to have on any electrical system, they have their limitations when used as the sole device to monitor battery charge level, which I proved out last season.  Going to bed one night tied up to the dock the volt meter was reading 12.3V.  Before tucking in I turned off everything including the switch supplying the volt meter.   The next morning I woke up and flipped on the accessory switch and the meter was reading 12.1V, which indicates I was at about 50% capacity.   Hopefully everyone knows this is about the lowest you want to drain your deep cycle batteries to maximize their life.  This left me thinking...   See if you don't let a battery sit for a half day or so from either supplying a load or receiving a charge the voltage reading may not accurately indicate charge%.  In this instance the voltage was showing some surface charge after having run the outboard for a few minutes to pull in.  By morning it had fallen to show I needed a charge before any further major power usage.  This season holds plans for a few trips in the 4-8 day range.  I wanted to have a little better idea where I stood when the plan is to spend more than a couple nights in a row on the hook. 

In my research to find a better way to monitor battery charge level than a typical volt meter I found many "smart" battery monitors that all had good reviews.  I also found this article that gives a very good in depth into how most of these devises work and the problems around programing them https://marinehowto.com/programming-a-battery-monitor/  The other problem was all these smart monitors cost well in excess of $150.  I would consider one of these a must have on a vessel with a large battery bank that spends weeks off shore.  But I didn't want to spend that kind of money to monitor a single battery on a boat that sees only a handful of multi day trips a year!  Take out all the smart features of these devises like measuring temperature, applying a Peukerts constant, charge efficiency... and with a couple exceptions, most do the same basic thing.  Take the amp hour capacity of the battery bank, keep track of draw and charge amounts, and show the difference as the state of charge level.   

When thinking about my setup I came to these conclusions.  First, my battery always starts out a trip at 100% as the onboard charger is always plugged in between trips.  Second, the only thing I have to charge the battery with while off shore is the outboard.  Normally that doesn't run for more than a minute or two unless there is no wind.  When I do run the motor for extended periods I don't normally have any high draw devises that are kept on, usually just the depth finder and maybe a phone plugged in.  So even at lower throttle its little charging system will easily supply power to my needs, plus put some charge in the battery.  So that takes one major feature of the smart monitors off the table as I'll never have more amps going in than out unless the outboard is running, or I'm plugged into shore power.  So I don't need a meter that can constantly keep track of both discharge and charge rates to calculate the net discharge amount.

I found a meter that shows volts, amps, watts, and watt hours on Amazon for $18.75  https://www.amazon.com/gp/product/B013PKYILS/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 It appears this unit is re-branded by a few different companies and is widely available through many outlets.  It also showed up on more than a couple lists of recommended battery/charge monitor systems I found.   It can be wired with a DPDT switch to either show discharge or charge rate.  Or if you don't want to re-set the watt hour count and flip a switch all the time, or you have a solar panel or other charging devise that may not keep up with your loads at any given point, you can put in two of these meters, one to monitor draw, and one to monitor charge, like it shows on the link.  Or if you don't have any way to charge onboard just wire it direct to show discharge.  I found many other meters but this was the only one in the price range that showed cumulative power usage.  It is more precise and accurate than other cheap meters I've seen.  The volt reading is to the hundredth, and compared to my Fluke multi-meter both volt and amp readings are within 0.2%.  It will be nice to see amp consumption of all my electronics to better estimate my usage requirements down the road.  I also like that all of the information is shown on one screen and you don't have to flip between screens to see all the data.

I did a full draw test on my 105 Ah battery and found the actual capacity to be darn close to 100Ah. (See article link)

I don't mind doing math, especially when it saves me a couple Benjamins!  So I figured I can keep track of the variables close enough to be comfortable with the figures.   To keep things simple I'll use 12 as the factor for converting watt hours to amp hours and consider I have a 1200 what hour battery.  To explain in more detail... A 12V lead acid battery at above a 50% charge level will put out more than 12V, so my battery will have more than 600 watt hours available in the first 50% of its amp hour capacity.  Also, because most of my power usage is well below the 20 hour amp rating (5amp) of the battery, Peukert's law shows I will have a bit more Ah available than the batteries rating.  These two factors should make up for any other losses and keep my calculations showing a slightly higher discharge total than actual.  If I never get below 600 watt hours I'll assure I always maintain a maximum 50% draw.  When charging the actual voltage will also be over 12V, using the 12 dividing factor when charging will also leave me with calculated values higher the actual, and in this case that's not good!  So I'll be very conservative with charge efficiency and apply a 75% efficiency factor to make up for that and any charging efficiency losses. 

Here's an example of a theoretical weekend with my 1200 watt hour battery. 

Day one, discharge rate 200 wh, charge 0 wh
1200-200 = 1000 = 83% battery capacity remaining
Day two charge 270 wh during day, discharge 0,  no wind, motor cruised all day:(
1000+270 X .75 = +2.5, fully charged, re-set at 100%
Day 2 evening/night discharge 250
1200-250 = 950 = 79% capacity remaining
Day 3 discharge 200 charge 20
950-200+20 X .75 = 780 = 65% capacity remaining.  Time to get her home and plug her in!

I know these numbers don't represent anywhere close to what my actuals will be, it's just for demonstration purposes.  With my LED anchor and cabin light I hope to get at least a few nights in without needing to put any charge to the battery.

A note on outboards with unregulated charging systems.  If you're running the motor for extended periods keep an eye on voltage.  With the low amps 6-9hp outboards put out the battery will typically regulate voltage by itself, just make sure it's staying under 14.8V (assuming wet cell).  Turn on lights or some other load to keep it in check if necessary. 

I'm hoping this addition alleviates some anxiety, extends my battery life, and limits any unnecessary motor run time.

Fair Winds!

[Bristol14]

Excellent tip and find. It also looks like there are similar or the same devices under other "brand" names that are even cheaper ($15.20).

I also saw a solar controller/regulator that looked like another bargain ($11.99). Here's the link:

https://www.amazon.com/Controller-Paremeter-Adjustable-Display-Regulator/dp/B06W5NP5JR/ref=cm_wl_huc_item

Has anyone used this?

[slode]

Just be cautious about the cheaper options as not all of them come with the shunt.