I live in an area where it is very cloudy most of the year, but dry and sunny (and sometimes very hot) for three months of the year. In order to get adequate solar power during the cloudy months, I have to have about 5X the solar panel area that I'd need in summer. My question is what to do with the excess power in the summer. My needs are pretty basic: refrigerator, lighting, charging a few electronic devices. I don't need or want to add more electric luxuries. My thought was to add Peltier thermoelectric cooling to the battery box, which should improve battery life(it can get up to ~44 degrees C /110 F here.) Peltier coolers aren't efficient, but they are by far the simplest cooling method, and they are cheap on ebay. I would use a thermostat and voltage level detector to make sure that they only operate during the sunniest/hottest part of the day. Has anyone tried this? Is there any downside? If so, what size Peltier cooler did you use? Any other thoughts on using the excess solar power?
Battery cooling from excess solar power?
- Thread starter Step-Hen
- Start date
dwh
Tail-End Charlie
So if I read this right, you're just looking for a way to burn up some excess watts that you could be harvesting during the summer?
You could heat water.
As for the peltiers, they generally need a heat sink and fan on the hot side to really work properly. If you had heat sinks and fans, you could cool the batteries with those and wouldn't need the peltiers.
Lead-acid batteries work better when warm anyway, up to a point, which is why a tempurature compensating charge controller will back off the charge voltage.
Thermal skocking the batteries by creating cold spots with peltiers might have a negative impact on battery life.
Running your solar array at 1/5 its rated output might have a positive impact on array life.
If it was me and the temp was 110f...I'd be more interesred in cooling myself than the batteries.
You could heat water.
As for the peltiers, they generally need a heat sink and fan on the hot side to really work properly. If you had heat sinks and fans, you could cool the batteries with those and wouldn't need the peltiers.
Lead-acid batteries work better when warm anyway, up to a point, which is why a tempurature compensating charge controller will back off the charge voltage.
Thermal skocking the batteries by creating cold spots with peltiers might have a negative impact on battery life.
Running your solar array at 1/5 its rated output might have a positive impact on array life.
If it was me and the temp was 110f...I'd be more interesred in cooling myself than the batteries.
Not looking for a protective dump load
I probably could have been more clear- I'm looking for something useful to do with the excess energy, not for a dump load like you would need for wind power.
I probably could have been more clear- I'm looking for something useful to do with the excess energy, not for a dump load like you would need for wind power.
Water heating is # 2 on my list. Evaporative cooling for me is third.
I don't think thermal shock is a big issue, since most thermoelectric devices can cool no more than ~20(?) degrees below ambient, and I would be using a large heat spreader (cold spreader?) [edit] I found a good article on battery thermal management that states "A temperature gradient across the battery pack can seriously affect the life of the battery. Arrhenius' Law indicates that for every 10°C increase in temperature, the chemical reaction rate approximately doubles. This puts an unbalanced stress on the cells in the battery and also exacerbates any age related deterioration of the cells." So it sounds like a heat spreader is essential.
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Yes, exactly! I'll even go further and admit that sometimes I enjoy the process more than the result. But seriously, if the problem is the expense of replacing batteries every few years, extending the lifetime 2X by eliminating temperature extremes isn't pointless. My initial plan was to shade the batteries while allowing air circulation, but while that will keep the batteries about 15 degrees cooler than when in direct sun, they still get far above their 25C design point on hot days, and stay that way well into the night (I used a cheap temperature data logger to verify this.)
That's what I was thinking. I have some reflective, closed-cell plastic insulating material I can use to wrap the box. It's effective enough to keep the batteries cool until several hours after the sun is up, so the thermoelectric cooling only has to be active for a few hours at most. My concern with insulation was that it would trap heat generated by the batteries if the cooler fails, but by my calculation the batteries will only produce about 26W of heat at maximum current draw (160A,) so it's not likely be be a big problem- I will rarely need to draw that much current for more than a couple seconds.
Seriously? Unfortunately, I can't post links, but here's a quote from Battery University: "As a guideline, each 8°C (15°F) rise in temperature cuts the life of a sealed lead acid battery in half." The PowerSonic Battery Technical Manual (page 11, Figure 11) shows that at 25 degrees C, the expected life of their AGM batteries is ~6-9 years, and at 40C it's 3-4.5 years. Trojan isn't quite as quantitative, but they do state in section 3.11 of their User's Guide that "Elevated battery electrolyte temperatures of >80°F (27°C) will reduce operating life." Check out the articles on mpoweruk dot com for more technical information.
And if you could get 10 years instead of 6 for $40, why wouldn't you?
If you are daily cycling to 50% DoD, you're probably looking at around 1,000-1,200 cycles, which is 3-4 years.
To even hope to get 10 years, you'd need 3600 cycles, which would mean 10-20% DoD max.
Good points, but I guess it would have helped the discussion if I had given more information about how I'm using the batteries. There's no way I'll ever cycle the batteries to death under normal circumstances- I chose the battery capacity for maximum 50% DOD, and will use them camping maybe 2-4 weeks per year. The rest of the time, they sit around on trickle charge waiting for TEOTWAWKI (there's a 1 in 3 chance of a magnitude 8+ earthquake where I live in the next 50 years, 1 in 10 for a 9+ quake.) Estimates are that power would be restored within 3 months in the event of an earthquake, so even for emergency backup purposes, I won't cycle them to death.
PhulesAU
Explorer
Seriously, if you batteries will be used under laboratory conditions that might mean something. When you buy 4 at at a time for a fleet of trucks, and use that kind of testing it will be more useful.