Three Batteries

Big Duck has three batteries.
Start
House
Inverter
These are three different batteries. The start is a 24, the house is a 27 (both wet) and the inverter is a 31 AGM.

I charge all three from a single charging source via 2 VSR's from Yandina.
Yandina VSR
When the Yandinas are in the AUTO mode, any charging source will be distributed to all three batteries. This charging source can be:
Onboard charger
Inverter charger
Engine alternator
Experts will tell you that this is not a sound practice - only batteries of identical size and age should be charged by the same source. I'm violating that rule on several accounts for the sake of economy and simplicity.
My take on batteries

Most of the time I have all the batteries combined via the Yandina AUTO function and because I'm so lackadaisical about my charging methods, I needed a way to see if one of them is heading south.

My first thought was a single voltmeter with a triple throw switch that would show each battery one at a time. Then I stumbled on these little gems on ebay...three for under $25.

So then all I needed was a lead from each battery to a three pole switch (because voltmeters will draw a battery down) and then to these voltmeters. These switches are readily available.

 

So now, if I set the combiners to OFF, I can see the voltage of each battery and how it's holding up after a load is applied.

 In the Yandina combined position, of course all the meters will show the same voltage if a charging source is applied, but if the Yandinas are forced off and no charging source is present I can get a feel of how each battery is holding up.

Would the right thing to do be having all the batteries identical and of the same age? Well maybe, but until then, I'm covered.

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Here's a comment on this approach from another board:

So I read Jeffs article and disagree with his modus operandi. Firstly, deep cycle batteries charge differently than start ones, secondly, different types of batteries should not be charged on the same cycle and thirdly AGMs take charge at a higher voltage and in most cases should never be equalized unless okayed by the MFGr. The Yandina combiner is basically to keep a troll motor battery charge, not for continuous use. More importantly, an external regulator is of essence on a boat which keeps the batteries at an optimum charge from variable sources.  

And here is my response:

Your point is well taken and I'll be the first to admit my approach to batteries is not ideal. But...my boat is over 40 years old and for some of us ‘bottom feeders’ the ideal approach is not always the best - we always look for something that just ‘works’. We know what the books say and how the serious boaters do it, but we opt to take a more casual approach. Before I bought my trawler I wondered how I was ever going to keep up with the high tech advice I got from every board. It was absolutely overwhelming…must have this…must do it this way…can’t be without this…this is what the books say -- and none of that ‘encouragement’ was cheap! I bought the trawler anyway but it was so much responsibility that it wasn’t any fun (even though it didn’t cost me a cent because it paid for itself by being in charter service). I found I didn’t want to have to worry about all those expensive elaborate systems. 

Today we have all the comforts of the trawler with none of the worries. She sleeps 7, has 2 mechanical steering stations, double galley sink, 4 burner stove and large oven, air conditioning and heat via a remote start Honda generator on the upper deck, microwave, fridge, freezer on the aft deck, hot water shower, gravity toilet, reliable 350 Chevy power for 25+ mph -- all simple systems that work day in and day out whether on the water, on the road or on the trailer in the back yard as a guest house. I’m certainly not knocking the folks who want to do it right, but if it ‘works’ and will get me out on the water, that’s good enough for me…and maybe also good enough for lurker/dreamers who hesitate to get their feet wet because they can’t manage to go the ideal route. 

My purpose is to encourage other (maybe lurking) simple folks to take the plunge. If you want to cross oceans, then listen to the big guys. If you just want to have some fun on inland and coastal waters, well maybe some of that high tech stuff isn’t all that important?