AGM voltages

Hi David,

I can't be very specific about AGM, because my experience is confined to wet cell and gel cell types, however the chemistry is very similar (voltages are a bit different), so my experiences may throw some light on your situation.

First thing is to establish just what your load actually is. My 40 litre Waeko consumes about 3.6 amps when running, and I reckon (educated guess) that it probably runs for about 30% of the 24 hour day, hence in a day it consumes about 25 amp hrs. How big is your flouro? A little one, with 2 tubes 9" long, will consume about 1 amp, runs for say 3 hours at night = 3amp hours per day. At this point it's worth noting that 90% of the load is the fridge. My battery also charges the laptop, cameras, phone, and runs the hf and cb radios, but in the daily tally they don't take much - the fridge remains by far the major load. Your Fridge is bigger than mine so probably takes a bit more.

On the above basis, I require at least 30 amp hrs, possibly 35 Ah each day. You probably need 35 - 40 Ah

Batteries - you've already been to the windsun site (http://www.windsun.com/Batteries/Battery_FAQ.htm),
which is the most informative source of which I'm aware.

I've done discharge tests with my 130 Ah wet deep cycle battery, applying a constant current load and monitoring voltage as the battery discharges. In my case there is a roughly straight line decline from the nominal 12.6V down to about 11.5V, accounting for about 100 Ah, then another straight line decline (much faster) from 11.5 down. For this reason I regard 11.5 as my preferred end point, and 11.0 as the point where any non-essentials (eg cold beer) are cut off. Battery life is substantially reduced if the battery is discharged too much further than this, especially if it is allowed to remain deep discharged for long. My key finding here is that you actually get only about 75% of the nominal battery capacity before you reach an end point where the rate of voltage drop becomes quite high.

In your case, I think your battery has probably provided about 60 to 70 Ah during its 40 hours operation, and so I would expect it to be approaching a conservative end point.

Regarding the fridge cutout voltages V's measured battery voltage - there will be some voltage loss in the cable and connectors (resistance) when the fridge is drawing current (see ohms law), so the fridge isn't seeing the full battery voltage. This is particularly true during starting, when the fridge draws much higher current than it does when running. Note too that the internal resistance in the battery means that the terminal voltage depends on the load current too, so measured voltages will vary with load.

Suggest measure currents and get a better idea of just what your load actually is. I'd be a bit disappointed with the apparent battery capacity, but really need more data to suggest that there is anything wrong.

HTH

John

Hi David

I have a graph on my site that can help you work things out.

The graph is showing capacity while under load. This will help your question to be answered. Remember your fridge measures voltage internally and your other meters are external direct to the battery.

SOC Graph

Regards

Derek.

Is the 12.38 volt battery reading with out a load?
i.e. fridge not connected or not drawing current?
If yes, maybe the battery drops to under11.5v on load with the fridge connnected and drawing current, and so cuts out?
Have you put a multi meter on it while the fridge is not drawing and while drawing to see if there is a difference?

If your battery guage is connected at the battery, and your wae, ...waco... , waeco (Sorry I have an engel and find it hard to spell these funny names :O))) cutout is set at 11.5v , how much voltage drop are you getting in your wiring from battery terminal to fridge terminal? About 1v? Suggest you get the multimeter out again for a check.

The angle of the engel when dangling is generally 90 degrees, plus or minus 30 degrees for it to work properly. :o)

Not sure if you can pull the plug back at the fridge a little to get the point of the multimetre in there. Otherwise if it is simple enough, take the cover off to pick up the terminal at the back of the plug? Or can you get a reading at the fridge side of the car plug, next best? Reading with fridge running and not running?

Hi David. I'll be watching the replies here. In the same boat.

Just got a 100ah Remco myself and Waeco 50lt. I ran the fridge in the lounge set to 4 degrees (pre-chilled from 240v). It measured 12.91 at rest before I plugged the fridge in and measured with a volt meter the at rest voltage every 2hrs or so during my waking days, and planned to draw a time/volt graph to see how long it took, and where the voltage sat for periods. I had to stop tho, cause I went away by the time it got to 12v and had to disconnect. This was nearly 5 days later too. The volts were less under load that at rest.

David,

The graph you have identified from the Windsun site is for all lead acid batteries.

An AGM battery is merely another form of lead acid battery and therefore the graph is suitable for them as well.

Hi David

I guess it can be a bit confusing with different answers and tables, but an AGM battery is different from your normal types and has generally higher voltages.

The following table shows both Normal and AGM state of charge voltages.

http://www.uuhome.de/william.darden/SoC.xls

Robin Miller