Originally Posted by Limey Linda
I agree with JW, keep it simple. My experience with two batterys is mainly in an off road vehicle with a diode. However, although this system has saved my A. on numerous occasions, there is a down side. It tends to lull you into false security unless you check the condition of the 2nd battery regularly. Ok, so it still starts but will the 2nd bat. save you if the primary dies? The voltmeter will show everything is Ok even if the 2nd battery has failed. I am going to install a two switch system with no diode and a voltmeter across the common neg. and a point between the alternator pos/two switches. In this way I can quickly check condition of each battery and the combination; no load and loaded and disconnect either one if they look dodgy. Normal running would be both batterys connected. House supply would be from the 2nd battery.
That's exactly my point. You can't assume a diode (alone) is keeping the 2nd batt in a condition where it will start your motor. It probably will but you can't guarantee it 100% unless you monitor it regularly. Sods law dictates that when you need it most will be the time when it's not going to work (as has just happed to me with my jump starter). In the case like JW describes (ie 2nd batt always being fully charged) this will work but I like certainty and if for any reason the 2nd batt gets discharged and you don't notice.... anyway we've been there...
So - why bother with the uncertainty? I am all for simplification. I like switches and have used them in the past so I can selectively charge batteries independently but most of the time when engine was running the batts were linked so they all charged at once. This is already a good system! An enhancement to that is fitting something to link them automatically - either a diode or a charge relay. Charge relay has the advantage that it will fully charge the 2nd batt from any state and is pretty much fit and forget (Caravaners have been using them for years). This removes the user intervention of forgetting to switch (or more likely de-switch) the batts. It also covers all bases for use of the batt's. Agreed, a backup batt is a slightly different use case but again we've been there....
If you were really into the fault tolerance / safety critical side of things, you could either fit a second voltmeter on the 2nd batt or fit an LED to show it's charging and assume it's the same voltage as the main batt. Again a bit of overkill but if it's a real safety concern, it's a small price to pay.