I naively thought this would be a quick and easy project but it turned out to be a bit of a minefield, the more I learned about battery types, split charging, battery monitors etc. It has been a bit of a journey, and I think I've learned a bit along the way. I don't profess to be a battery guru, but I certainly know more than when I started. Hopefully my experience will help others with questions about this topic.
The choice of battery type should have been simple enough. With my rudimentary knowledge of batteries, I knew I wanted a deep-cycle battery as big as I could fit under the bunk in the rear cabin. For the uninitiated, deep cycle batteries are designed to deliver power more slowly over longer periods than the typical starter battery, which delivers a short high burst of power. Deep cycle batteries tend to have thicker plates and can recover from deep discharging more frequently. Deep cycle batteries come in several different types: wet-cell, Gel type and AGM. The Gel-type and AGM batteries appear to be better at deep cycling than wet-cell batteries, and I initially thought I'd go for one of these, but they require different charging voltages (and cost a lot more!). This is where it started to get more complicated than I expected!
Most modern chargers have different settings for the different types of battery, but what would happen if you try to charge different battery types off the same charger at the same time? You run the risk of overcharging (and damaging) one of the batteries because of the different charging voltages.
I researched as much as I could on the Internet, but could not get to a satisfactory solution. Some sites suggest using 2-stage chargers rather than 3. In the end I was not left feeling comfortable about the various ideas so opted to keep the battery type the same as the engine starter battery (wet cell); this way the charging voltages would be the same.
The next challenge was to figure out how to charge both batteries off both the alternator and the shore-powered charger whilst keeping the circuits separate. In simple terms, the battery circuits need to be isolated when in use (to avoid discharging the starter battery), but connected when being charged (from either charging source).
The great thing about the Internet is that someone has always done it before; I started to learn about the concept of split-charging.
There is a simple way to achieve split charging i.e. you could use manual switches, but you need to remember to connect the battery circuits when charging and remember to disconnect them when charging is complete. Forget do do so and you could well end up with a flat engine battery when you least need it. As we view ourselves as "gin-and-tonic" kind of sailors, this would be a recipe for disaster!
There are a number of solutions commonly used and I'll try and explain these as best I can:
Split charging diodes
with a 12V battery. This means that the batteries don't see the required voltage, charge slower and may not fully charge.
Split charging relays
These are electrically operated switches that will connect/disconnect the charging sources (alternator or shore-fed charger) from the batteries depending on whether they are running or not. This solution needs 2 relays - one powered by a contact on the alternator and one powered by the shore supply.
The relay option seemed to be the best although more complicated in terms of wiring.
Around this stage I 'd been researching battery monitors as I wanted a way to estimate how depleted the domestic battery was so that it could be recharged when appropriate. I was hoping that something fairly simple like a voltmeter would do the trick. Wrong again!
|State of Charge versus Voltage|
This chart shows the battery state of charge for a given voltage. As you can see, a fully charged 12V battery actually has a voltage of around 12.7V. When the battery is around 50% discharged it has a voltage of approx. 12.2V. For most lead acid batteries, discharging the battery below a 60% state of charge is apparently not good for the longevity of the battery, thus using a simple voltmeter is not going to be a good idea. A further complication is that the battery "ages", for lack of a better description. In other words, it's capacity reduces with each successive discharge.
There seem to be 2 main types of battery monitor - those that count the Amp hours used, a bit like a domestic electricity meter and then estimate the charge remaining and a second type, that uses some clever electronics and algorithms to build a history of the battery from it's charging/discharging profile and estimates the capacity from this.
I must admit to falling for the sales pitch and going for option 2! I opted for a Smartbank/Smartgauge combination. This is a combined battery monitor and split charging system with some clever electronics thrown in to manage the whole lot. The beauty of the system is that it costs marginally more than a split-charging relay set-up with a separate battery monitor, but has simpler wiring, only one relay and has some nice-to-have features like being able to use the domestic battery to start the engine in an emergency. It also has contacts to allow you to trigger an alarm to warn of low battery state.
Sounds like the fit-and-forget type of kit that a "gin & tonic sailor" needs!
Here is a photo of the key components: this shows the control box or "Smartbank", the rather heavy duty relay and the battery monitor or "Smartgauge"