Solar Ready Auxiliary Battery/Charging system. Help!

This thread might be considered off topic, but I could sure use some basic advice from some of you smart guys out there.

I want to begin investing in a nice solar system, but I want to start small, and work up to the full package. As it happens, I am interested in a good auxiliary battery system to start. I see this as the "main battery" of my system (this would be te battery that will eventually be charged by a nice quality, flexible solar panel).

In the meantime, I simply want to be able to place this battery in my car, and recharge it from the cigarette lighter socket or some other 12V source powered from my small car's small alternator. I would like to think in terms of investing in a high enough capacity, high quality battery (gel cell?) to be truly useful.

My immediate goal would be to be able to provide reliable power to a good quality 12v Fridge/Freezer that could draw as much as 3.0 continuous amps on a very hot day in Freezer mode. So, the questions that immediately spring to mind:

- How large does the capacity of such a auxiliary battery need to be for it to be useful in my proposed application?

- Will my little car alternator be able to keep up? (In other words, how many hours might I be able to keep using such a Freezer while driving down the road, and hopefully recharging the auxiliary cell as I go.

- How many hours can I power such a Freezer off the auxiliary battery alone (Car not running.)? This depends I am sure on the capacity of the auxiliary battery.

- Is this a "dumb idea" to start with? (Get a smaller Freezer with less amp draw.)

- Etc.



Looking forward to all thoughts and comments that you care to provide.

Hello Rosoku Chikara,

Let's see now we can get started by looking at the worst case. 3 amps over 24 hours would come out to 72 amp hours. If you had a 100 amp hour battery and ran your freezer continuously you would have to recharge the battery every 24 hours.

If you charging system is capable of providing 75 amps of charging current to that auxiliary battery, it would take about 4 hours to bring it up to full charge allowing for about an hour of balance charging.

If your charging system is capable of 75 amps but you are running your car and the freezer and trying to charge the auxiliary battery all at the same time figure on around 8 hours to charge the battery back up.

Tom

If you have a 100AH battery and you pull 72AH from it each day you will be lucky to get 100 days of use out of it.
Also running the freezer off battery, whether recharging from solar or from alternator will cost you up to ten times as much as running it off AC.
Both the battery replacement and wear and tear on the engine need to be taken into account.

If vou want to get started on a DC freezer, get an AC fed 12V DC supply.
Finally, is this a compressor type DC freezer or does it use solid state, Peltier-Seebeck effect cooling? The latter is horribly inefficient.
On balance a bad idea.
Note that the common rule if thumb for long battery life is to discharge at most 20% per day.
For 72AH of load that would be a 360AH battery.

I am considering (dreaming about) an Engle MR040 (40 liter) unit that uses a Japanese made Sawafuji "swing motor compressor" which is said to be extremely efficient. The 100% full-time 3.0 amp draw refers to a worst case scenario where you would be attempting to freeze ice in 113 F environmental temperature.

Used as a refrigerator in more normal weather, the draw should be more like 1.5Ah/Hr and only about 50% of the time, when the compressor is actually running. (But, if the unit is indeed capable of freezing ice under such harsh conditions, I would like to be able have enough power available, in case I ever wanted to try it.)

This size Engel unit looks to be the most practical. There are smaller (also compressor) models available, but they are designed primarily for organ transport, etc. and as just as expensive despite much smaller size.

While I have not taken even my first look at price, weight, size and capacity of an auxiliary battery, I am not necessarily afraid to invest in the right capacity battery or batteries to get the job done right. Is gel cell the right way to go?

I am not sure what is meant by an "AC fed 12V DC supply". Of course, I want a way to recharge any auxiliary batteries by AC power when staying over night in a motel, etc.

But, there would not be any AC available when driving the car, I don't believe. Or is the thought to install some kind of AC generator?

I would want to be able to use this set up in rental cars or other vehicles that don't belong to me, so higher powered alternator is not a practical option.

Anyway, please kindly keep up the comments/questions. It is a great help as I attempt to get my mind around the problem.

I think it would help ideas if we knew exactly what you are doing. Like day trips, weekend trips or just being out in the sand with no electricity at all. In my little plan for a grid outage for whatever reason, I only want to keep the refrigerator and a few hours of light in the house powered up plus flashlight battery charging.

You may wish to consider other battery chemistries such as Lifepo4 as lead acid batteries are heavy and do not handle deep discharge well (as noted above). I haven't had experience with Lifepo4 but understand that they are lighter (more portable), handle deep discharges very well and have a long life. They are however more expensive. If you do choose a different battery chemistry make sure that you can obtain a suitable charge controller for that chemistry.

John.

lwknight said:

I think it would help ideas if we knew exactly what you are doing...<snip>

Click to expand...

As mentioned previously, I am simply trying to "begin" a project that would hopefully end-up as a nice Solar rig. My understanding is that a good auxiliary battery is to be found at the center of all such systems.

I am using the refrigerator requirement as the maximum anticipated load on the system. In an emergency or survival situation, I would not use the system to power refrigeration or freezing.

I am interested in the fridge/freezer for roadtrips, camping, pier fishing etc. A good ice chest would work fine for such applications (and at a tiny fraction of the cost), but I have always kind of wanted a good 12v fridge. And, if it can actually freeze ice, it seems very interesting to me. But, only for play, not emergency.

The whole thing (even the completed solar system, when eventually completed, needs to be easy enough to pack into a car.

I have noticed that large capacity Gel cells get very heavy in a hurry. I might decide to use 2 Gel cells, say 100 Ah each. A 200 Ah battery looks to be quite heavy. (If so, I could start with only 100 Ah and add another 100 Ah someday in the future.)

jcs0001 said:

...Lifepo4 but understand that they are lighter (more portable), handle deep discharges very well and have a long life...<snip>

Click to expand...

I also took a quick look at LiFePO4 and they have considerable appeal, but at first glance I could not see how to easily achieve a 100 Ah capacity, much less 200 Ah. Perhaps I am missing something?

Regarding lifepo4 - here is a link to a good article about a 180 amp hour (12.8 volt) lifepo4 battery.
http://www.fieldlines.com/index.php/topic,130173.msg846789.html#msg846789

The source of the battery is mentioned at the beginning of the article. (not sure if this link is allowed but if not pm me and I can send you the link).

Regarding lead acid cells it is my understanding that you want to match such batteries - ie you want to have batteries with the same age and use together. Thus buying one now and another a year or two later (particularly if you use the first one extensively) will lead to having unmatched batteries which is not good.

More info regarding gel cells:

Gelled Electrolyte Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries.
Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death. It is for this and other reasons that we no longer sell any of the gelled cells except for replacement use. The newer AGM (absorbed glass mat) batteries have all the advantages (and then some) of gelled, with none of the disadvantages. (info from: http://www.solar-electric.com/deep-cycle-battery-faq.html).

The above noted info on gel cells indicates that planning to "quick charge" them may lead to short life.

John.

Another article about using lifepo4 as a solar battery:

http://www.fieldlines.com/index.php/topic,130167.msg846689.html#msg846689

The above has a very good discussion of lead acid (deep cycle) vs lifepo4 batteries. He used A123 system batteries which are likely not available anymore however other ones could be substituted.

John.

Nice - I want one!

Rosoku Chikara said:

The 100% full-time 3.0 amp draw refers to a worst case scenario where you would be attempting to freeze ice in 113 F environmental temperature.

Click to expand...

Ok, with the battery as a buffer, I'd recommend at least a 60 watt PowerFilm panel. I use Powerfilm panels myself, and noted that on the site in Australia, they endorse these. Good choice because they are not the typical camping-junk you see elsewhere. Sadly, I don't see them even mentioning solar on the USA site. What I don't see as an accessory is the all-important charge controller. This would be an "RA-9" controller that is good up to 4.5A input. The RA-9 is actually a Morningstar-Sunguard 4.5A model with special connectors that mate to the Powerfilms.

The reason for a 60 watt panel is that when the fridge is pulling 2.75A worst-case, the panel is capable of supplying about 3.3A (60w / 18v panel = 3.33A), thus you won't be draining the battery at all under proper sun. I don't know why Powerfilm specs their panels as 15v (most likely a nominal value) as they DO go up to 21v or so, and a controller is mandatory! When the fridge isn't operating at full-bore current, what's left will go into charging the battery.

Since you have alternate power sources available, doing a perfect calculation that takes into account the exact power draw over time, your solar-insolation hours (these are different than mere sunrise-sunset hours), perfect battery capacity calcs etc. Although ideal, we can kind of just go overboard a little.

Battery? Sealed AGM for sure. And in this case, a "tppl" or thin-plate-pure-lead battery like Optima or Odyssey would be a fine fit. But how large? It depends on how long you want to run. And being tppl, there are no charge current limits as long as you are voltage regulated, so that would accomodate as much power as you can throw at it, solar or otherwise. Ups-style agm's, like Powersonic, Universal battery and such have a 0.25 to 0.3C maximum inrush current, and that could limit you.

Lets assume 8 hours at 2.75A worst case. 8 * 2.75 = 22ah, and since you don't want to drag the agm below 50% capacity on a regular basis, that would mean you'd need a 44ah battery. But that is without the help of solar running from a fully charged battery in the first place when you start out.

Personally, I'd be looking at a 60 watt powerfilm, an RA-9 charge controller, and something like a small Optima yellow-top D51. Or an Odyssey PC-1200 about the same size.

Unlike ups-style agm's, tppl batteries don't have any current limit (as long as you are voltage regulated), so you can throw a lot of higher powered charge sources at it, like from your alternator.

When you get home, toss these on a DECENT charger, not just a little 1A battery tender. I personally don't use Schumacher speed-chargers, since they have a built-in tendency to go way over-voltage despite what the voltage readout on the display says.

Unless you want to get into deep IUI charge profiles and such yourself, the easiest thing to do is use an Optima Digital 400 or 1200 charger. Or Odyssey's chargers. Despite naysayers that like to use Grandpa's old charger from the farm, these really do the right thing. At least a Fluke 87V and other assorted gear that I have prove it.