Yes, if you’re getting down to 11.7v then you must be running your batteries right down to the limit (and/or have some really nasty voltage losses on your cabling). Lithium would give you more Ah-for-Ah capacity. Also, sticking in another AGM would do the same. Like every other scenario, carefully assessing the cost/weight/size/capacity trade-off in your application should make it clear the way to go.
What Leisure Battery Have You Got?
- Thread starter Dellmassive
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Yes, if you’re getting down to 11.7v then you must be running your batteries right down to the limit (and/or have some really nasty voltage losses on your cabling). Lithium would give you more Ah-for-Ah capacity. Also, sticking in another AGM would do the same. Like every other scenario, carefully assessing the cost/weight/size/capacity trade-off in your application should make it clear the way to go.
The voltage drop from the battery to the fridge would only have to be around 3% for a healthy battery at 50% charge (12.05v) to drop below the 11.7 volts - I'm not sure if 3% voltage drop is a reasonable figure but it doesn't sound too bad to me? Even if the drop off was only 1%, the fridge would only keep running down to around 30% of battery charge.
As far as I'm aware the battery (75ah AGM) is in good health and kept topped up via solar - unfortunately there is no room to put a bigger AGM in. When the conversion and van were brand new I was disappointed that the fridge would only keep running for around 2 or 3 days off grid even though there was still plenty of juice to keep my lights (including the light in the fridge) and other things running - this was my main reason for going solar, which I am very happy with and enables 24-7 running of the fridge without EHU. However, when it comes to replacing the battery I will definitely look at Lithium.
This is sounds like a considered analysis of the trade-offs involved, which is all I was really advocating in this thread!. As it happens, I have a very similar set-up with a factory 75Ah AGM. We were also wanting more off-grid time and so fitted a 250w solar panel to the roof which effectively enables unlimited off-grid time (well, until the water runs out!). If we really get stuck, we can just run the engine for a little bit to top it up from the alternator but we've never needed to in practice. I'm happy with my set-up so I'd only consider a lithium replacement if/when the price falls substantially by the time my AGM is knackered.
The only bit that's inconsistent with my experience is the 12.05v at 50% charge, my AGM battery has a substantially higher voltage at 50% SOC than that. Maybe your fridge is more powerful than mine and so draws more current and thus causes more voltage sag under load? Could it be at lower than 50% SOC at that point?
To be perfectly honest my only knowledge in this area comes from trying to understand why my fridge stopped working after a couple of days when there was still plenty of juice left. All of the charts I looked at for AGM batteries suggested 50% charge is around 12.06v which is what I was basing my figures on above - I have no idea at what state of charge the voltage was actually getting below 11.7v and the compressor would stop starting. Things were complicated further when I realised that ambient temperature was also playing a part and another factor in the compressor not running, i.e. sometimes when I thought the fridge had stopped working due to insufficient power were down to ambient temperature getting below about 5c and the fridge, by design, not running
To be perfectly honest my only knowledge in this area comes from trying to understand why my fridge stopped working after a couple of days when there was still plenty of juice left. All of the charts I looked at for AGM batteries suggested 50% charge is around 12.06v which is what I was basing my figures on above - I have no idea at what state of charge the voltage was actually getting below 11.7v and the compressor would stop starting. Things were complicated further when I realised that ambient temperature was also playing a part and another factor in the compressor not running, i.e. sometimes when I thought the fridge had stopped working due to insufficient power were down to ambient temperature getting below about 5c and the fridge, by design, not running
Err yes, temperature is going to complicate matters! The charts I could find, for example, this one:
Battery voltage and state of charge - Energy Matters
Imply that at 50% SOC, the resting voltage of an AGM is around 12.3v. This is broadly consistent with my experience of my battery from memory, but it's been a long time since it got down to 50% (hooray for solar panels!). The fact that the draw from the fridge will cause the voltage to sag is another complication but my fridge only seems to draw about 3amps so I can't imagine the voltage drop is too severe. I guess it's important to work out at what SOC the battery really is failing to drive the fridge because if it's much lower than 50%, then the benefit you get from swapping to lithium might be lower than you think.
At some point I'll try and actually leave the fridge running with the solar off and a voltage logger attached to get a detailed picture of voltage in practice versus SOC. Not much point doing it now though, it's so cold the fridge barely needs to be on! I'll let you know what I find when I do it though.
Agreed - it would make sense to understand at what state of charge the fridge stops working if using the "more available power from a lower Ah battery" as the main part of the lithium benefit equation (which I would to get "budget approval" past Jo
).For info; here is one of the sites I was using that gives contradictory information to that you posted re voltage and state of charge:-
Deep Cycle Battery | A Guide to the best Deep Cycle Batteries for Solar, Camping, Caravan & 4WDs
Agreed - it would make sense to understand at what state of charge the fridge stops working if using the "more available power from a lower Ah battery" as the main part of the lithium benefit equation (which I would to get "budget approval" past Jo
For info; here is one of the sites I was using that gives contradictory information to that you posted re voltage and state of charge:-
Deep Cycle Battery | A Guide to the best Deep Cycle Batteries for Solar, Camping, Caravan & 4WDs
Ah yes, that definitely isn't consistent with the link I had! I think we need actual measurements, I'll let you know when I've managed to get some.
Good luck with "budget approval", I frequently try and deploy man-maths to prove how my expensive gizmo is vitally important and money-saving. Rarely works, obviously...
That makes two of us then
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