Thumper 75Ah & Waeco 80lt

Hi guys,
I'm a bit new around here & love all your advice in general and I was hoping you may be able to help me with a problem I've encountered.
I have a 75ah thumper which I use to run my waeco 80lt. I've had myself up for the last few months and it's always worked well on the back of my Hilux tray-back. my problem arose this weekend when I took both of the car. The fridge switched off when the battery got to about 11.2volts. I have the fridge set to low so it shouldn't switch off till 10.2v according to the manual.
Any thoughts on if it is most likely to be the fridge shutting off to soon or the batter giving a wrong reading?? Just so I know which to chase up first.
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Reply By: chisel - Tuesday, May 22, 2012 at 13:28

Tuesday, May 22, 2012 at 13:28
If you let your battery go below about 11.5V (depending on what type it is) you will drastically shorten its life. So probably better to have the fridge cut out at the higher voltage.

Regarding the cutout - are you sure you've got the fridge on the 'low' cutout setting?
AnswerID: 486455

Follow Up By: Kate5 - Wednesday, May 23, 2012 at 09:37

Wednesday, May 23, 2012 at 09:37
Fridge def set to low double checked that.
FollowupID: 761787

Reply By: P2D2 - Tuesday, May 22, 2012 at 17:58

Tuesday, May 22, 2012 at 17:58
When the fridge started the battery would dip to 10.2V and cutout the low voltage switch.
75Ah thumper under most weather conditions won't supply sufficient power to run the 80L fridge for 24 hours.
AnswerID: 486478

Follow Up By: David16 - Tuesday, May 22, 2012 at 19:11

Tuesday, May 22, 2012 at 19:11
I've got a 75amp thumper, I only get 2 days running my engel 40 before i have to hook up the solar panel, they draw less than the waeco so I would say it will run even less on the 80. Either get a105amp battery or a $400 120 watt solar panel

FollowupID: 761735

Follow Up By: Kate5 - Wednesday, May 23, 2012 at 09:48

Wednesday, May 23, 2012 at 09:48
the battery ran the fridge on -10 for about 25 hours at about 15 - 25 oC ambient temp which I was pretty impressed with . I'm not sure how long after the 25 hours it lasted as it was afternoon when I went back to it about 5 hours later. It was just the cutting out when the battery still had a bit of charge in it I was wondering about.. I'm really happy with the set up I have so won't be upgrading battery at the moment. just trying to get the most out of it so by the sounds of it its doing pretty good.
Thanks for the reply guys
FollowupID: 761791

Follow Up By: Evakool - Thursday, May 24, 2012 at 08:17

Thursday, May 24, 2012 at 08:17
The image below shows the approximate state of charge of a lead acid battery shown by Voltage.

Ideally the battery needs to rest for an hour or more before you check the Voltage, but it can be used as an indication before this.

As per below, at 11.3 Volts there is only around 10% capacity left in the battery.

Also as P2D2 said, the Voltage of the battery will drop when the Fridge tries to start, there will also be some Voltage drop through any wiring inside the Thumper, Thumper to Fridge wiring and wiring inside the Fridge.
These two factors are probably combining to stop the Fridge on it's Voltage cut-out.

Image Could Not Be Found
FollowupID: 761878

Follow Up By: P2D2 - Thursday, May 24, 2012 at 08:58

Thursday, May 24, 2012 at 08:58
Evakool suggest you find a new battery voltage chart for AGM batteries which is what is in general use today. That chart is for older style lead acid batteries.
FollowupID: 761882

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