Leisure battery charge/discharge

[timhowes]

Hi folks

First run this weekend with my new 110Ah LB (Numax CXV31MF) and split charge/VSR installed. Running my shiny new compressor fridge with a safety cut-out (Waeco CF26). The fridge manual says it is safe to set the safety cut-out to "minimum" voltages when using a dedicated leisure batt - which means cut-out at 10.41. This sounds a bit low but I did it anyway to keep my beer cold! What do others think? Will I knacker the LB if I carry on using this setting? If so, have I knackered the LB already in one weekend?!

Next question: solar panels. I'm quite keen to have a slimline/flexible panel (rather than a framed one), mainly for aesthetics but also to keep clear from stuff on my roofrack while in transit. I can fit 100w of back-contact panels and understand these are less efficient than the frames. I also wonder about the need to replace a defective panel, and think this must be easier if said panel is bolted on rather than Sikaflexed. What are the experiences of others? Are these panels likely to go wrong or am I being paranoid?

Thanks for any info offered.
Tim

[g8dhe]

That is a bit low when its NOT being measured at the battery!
Mine (low voltage cut out relay) is set for 10.4 volts but its sat directly on the battery terminals, by the time you have some wiring, switches, fuses to the fridge you would be dropping another 0.2-0.5 volts possibly so that would mean that the fridge won't stop until the battery is 10.4-0.5=9.9volts and that will be damaging the battery I would try to set it for about 10.8 to 11.00 volts just to allow a little lea way!

I have a roof rack and a full panel sits below it in a frame with air clearence no problem; http://www.g8dhe.net/bongo_images/roof_bars-rack/

[timhowes]

Thanks Geoff,

I'll reset the cut-out to a more appropriate level. Is my battery likely to be damaged after half a day under such conditions?

Re the roof rack/solar panel conflict - I've already purchased a pair of simple bars that bridge the mount points (rather than the full-length rails that you have) and am more concerned that "lumpy" things on my roof bars might scratch/damage the panel - an adhesive model would be further from the items mounted on the roof. But perhaps I'm over-thinking the situation. Thanks for sharing your pics.

[g8dhe]

No its not an immediate death, it just damages the plates the more its done and the deeper the discharge, which is why its usually referred to only a 50% discharge.

[Jimbobvfr400]

I'm not convinced even the new higher limit would have been reached in half a day, a fully charged LB would easily manage at least 2-3 days of running your fridge.

[cmm303]

That is a bit low when its NOT being measured at the battery!
Mine (low voltage cut out relay) is set for 10.4 volts but its sat directly on the battery terminals, by the time you have some wiring, switches, fuses to the fridge you would be dropping another 0.2-0.5 volts possibly so that would mean that the fridge won't stop until the battery is 10.4-0.5=9.9volts and that will be damaging the battery I would try to set it for about 10.8 to 11.00 volts just to allow a little lea way!

I have a roof rack and a full panel sits below it in a frame with air clearence no problem; http://www.g8dhe.net/bongo_images/roof_bars-rack/

I'm missing something in this carefully thought out logic I would have thought that the voltage at the fridge will be less than that at the battery. Current through live cabling will cause a voltage drop from battery to fridge. Therefore 10.4 at the fridge will be 10.6-10.9 at the battery. Put another way the voltage drop across the fridge and cabling must ADD up to the supply (battery) voltage.

Do I need to get my coat?

[g8dhe]

Ah yes and no!
The low voltage cut out wants to operate BEFORE you start drawing current ideally, so that when the battery voltage does drop below 11 volts it stops the fridge working, otherwise when it starts to draw current the voltage will be below 11v approaching 10.4v. Hence setting it to 11 volts.
Yes I added the voltage drop calculation as an after thought and reversed the situation!
We can both get our coats, but its to warm down this way for coats, so lets leave it for next time

[cmm303]

Phew, I'm with you! Also I don't know how much hysteresis is designed into these cut out relays, but if its less than the voltage drop across the wiring then measuring the volts at the fridge could cause the cut out to keep cycling off-on-off as the voltage swings between the loaded and no-load conditions.

[g8dhe]

Yes that is often the problem, the volts drop, compressor cuts out, voltage rises and the cycle starts again, this is of course even worse on the battery as its the startup current every few seconds! If its just a low battery then the voltage won't rise as rapidly but if its caused by poor wiring connections etc. its the worse case scenario!

Its also the reason I have mine for everything located at the battery terminal itself, what ever causes the volts at the battery to drop then its cut out until charging starts again - Alternator/ Solar or Mains.

[timhowes]

Thanks Geoff and Chris, all seems to make sense. Fridge threshold reset and I'm sure all will be well from now on