This is another often overlooked yet important aspect of programming a battery monitor, if it even has this capability. This is one which both DIY’s and pro’s ignore with regularity, and neither installer really should.


A 100Ah battery rated at 77F will not deliver 100Ah’s at 45F….. "What? Huh? But it’s a 100Ah battery?" Yes it is a 100Ah battery but only at 77F and when discharged at the 20 hour rate of 5A. At any other temp or rate of discharge your usable capacity varies and this is what the Ah counter is trying to calculate for with the SOC screen.


Here in Maine it’s not unusual for me to measure batteries in the early spring at 40-45F as the water temps are keeping the hull & battery compartment at close that temp. I then go into the owners battery monitor, such as a Xantrex Link-Pro, and see it programmed for 80F or 75F etc.?


What do you suppose this does to the calculations & compensations the battery monitor is trying to apply in order to achieve the correct SOC for the battery? Do you think the Ah counter stands can be accurate with a 40F +/- difference between programmed temp and actual battery temp?


Many Ah counter makers miss the mark entirely and don’t even give you the option to program for temp nor do they offer an external temp sensor..


Ideally the best Ah counting battery monitor will be the model that offers an on-battery temp sensor so these calculations can be done in real-time. The Xantrex Link-Pro or the new Victron BMV-702 both offer this option as do Philippi Battery Monitors. Others may offer a programmable temp setting.


If a monitor offers a temp setting, as opposed to an on-battery temp sensor, the temp setting should ideally get periodically reprogrammed as water & battery temperatures change from season to season.


Unfortunately different lead acid batteries also perform differently under varying temps but most battery monitors do not allow you to adjust the slope for temp corrections to SOC. The Victron BMV-702, with optional temp sensor, is the only one I know of that allows slope corrections for temp. Why would this be important? A TPPL AGM will not be affected exactly the same way as a deep cycle L-16 Rolls battery would by temperature. Bottom line, if your battery monitor offers a temp sensor, use it...


Self Discharge:


While we are on the subject of the effects of temperature now is a good time to discuss self discharge. As most boat owners well know, a lead acid battery will lose capacity just sitting there. While some Ah counters ask you for a % each month during set up, this self discharge, like everything else, varies with temperature. Suffice it to say that an Ah counter can't really track self discharge very accurately. If your boat sits there for a week or two or threenone of the self discharge will be accurately counted.