Powering Jeep's House Bank

pebbledropper wrote: »

Please advise how best to use what's under my hood to help charge my house batteries.

My overland JKUR has 2 Lifeline CT-6's in series rated new at 300 12V AmpHrs, or 150 usable Ah at 50% DOD. On the roof rack are 2-160W panels with 18.5 Vmpp, 8.7 Impp, 22.2 POC, and 9.7 Ioc. The controller is TBD but will be MPPT. All connections to the batteries, other than the shunt and cat fuse will go through a buss bar.

Sounds like a nice setup. What are you loads you plan to run (i.e., peak Amps, Amp*Hours per day, etc.). Do you want to dry camp in one place and use solar panels to supply daily loads for days/weeks on end? Or do you plan on driving every few days to a new site, etc.?

I want to charge this bank also with either my alternator or the charging battery. I've read a few threads on this site about this subject. A separate DC-DC charger seems to be better because it might provide a higher voltage than the alternator will. I understand my AGM's want 14.8 as a bulk voltage. That level, of course, drains the starting battery faster than the alternator can replenish (I think). I can install the usual solenoid isolator; but the alternator will never send more than 13.2 to the bank and then even only after making sure the starting battery has all it wants.

For Lifetime AGMs, they recommend closer to 14.3 volts @ 77F:

http://www.lifelinebatteries.com/manual.pdf (page 19).

14.8 volts is pretty high nominal charging voltage (although, Lifeline batteries are supposed to handle higher charging voltages than other AGMs--Do more research here).

The only reason to bulk/absorb charge at higher voltages would be for quicker charging--Do you need to quick charge the battery bank?

Regarding the Alternator output--It should be much closer to 13.8 to 14.2 volts or so... 13.2 volts sounds awfully low (unless you are using a diode isolator--Which still makes the charging voltage for your house bank still too low). A solenoid isolator should not have as much voltage drop.

I can put the DC-DC charger inside by running heavy cables from the battery. That's not very far. I can install gauges and switches to let me turn it on/off, however, that's all still manual and more subject to error. At least, it won't have the protection of an isolator"s smart solenoid.

Here is a DC to DC battery charger / Isolator that may be of interest:

http://www.solar-electric.com/batter...h2/me-sbc.html

But, this simply connects the two battery banks in parallel (can use an external relay too). So--Still have to look at your alternator's output voltage. If it is closer to 14.3 volts (use a Marine Alternator with adjustable/deep cycle battery regulator). This might be an interesting choice:

http://balmar.net/

Another alternative is a true DC to DC charger -- Can take 12 VDC in and output 14.4 volts out (boost converter) vs the standard 14.2 volts in and 14.2 volts out of a "solenoid".

I would look at my house loads first... Figure out how many Amp*Hours / Watt*Hours you need per day first. That will tell us if the solar array will cover the whole load or not.

Next, how many hours per day you plan on running the motor--That would give us some idea of how much current you need to replace (i.e,. use 100 AH per day, Then 10 amps * 10 hours, or 50 amps * 2 hours, etc.).

-Bill

15.5 volts is very high from a typical vehicle alternator unless it is very cold outside (Florida is unlikely to get "that cold") unless the alternator has a remote voltage sensor (to allow for wiring/diode isolator drop).

Even 14.8 volts is pretty high for a sustained charging voltage on a vehicle.

Although, some newer vehicles are using higher voltage/more complex charging algorithms for battery charging these days (cars have more electric/electronic devices like computers, power steering, and air-conditioning pumps for government fuel efficiency regulations and such).

Here is an interesting survey from different manufacturers written around/before 2000:

http://www.landiss.com/battery.htm

The hybrid battery is more resistant to deep cycling than the lead/calcium, but is still not as good in this respect as the original lead/antimony style. Most cars supplied with hybrid batteries have their voltage regulators set to 14.3 volts, although it is reported that the higher 14.8 volt setting of GM cars does no harm if the water level is checked regularly.
...
The National Technical Director of the Mercedes-Benz Club of America (6) was considerably more flexible: "the specified voltage for Mercedes-Benz cars of the mid-1980s is 13.0 to 14.5 volts". But clearly 13.0 volts is not enough, and Bosch says 14.5 "will overheat and damage the battery". In an early 1993 letter (6) he says local MB technicians tell him that alternators usually check out around 14.2 to 14.4 volts, and that they replace regulators that call for less than 14.0 volts - thus contradicting the Bosch Product Support Manager. Are we having fun yet?

-Bill

Just to look at a "typical" setup... Using PV Watts for Fort Worth TX, panels mounted flat to roof, 320 Watts:



Month
Solar Radiation
(kWh/m²/day)


1
2.88


2
3.59


3
4.74


4
5.79


5
6.40


6
6.99


7
6.99


8
6.34


9
5.18


10
4.25


11
3.07


12
2.63


Year
4.91



Toss out the bottom three months... 3.59 Hours of sun for "break even" February:

• 320 Watts * 0.61 end to end system efficiency * 3.59 hours per day of sun * 1/12 volt battery bank = 58 Amp*Hours per average February day (12 VDC loads)

So--Looking at ~60 AH @ 12 volts. An MPPT charge controller will probably not help you much if you run around warm climates with those existing Vmp~18.5 volt panels (which work fine with PWM solar controller too).

-Bill

pebbledropper wrote: »

Usage-
My daily is higher than normal due to medical. I use a 12V snore machine and , if overnighting at altitude, a 12V O2 concentrator. The two of them require >6 Ah for ~8 hrs, or the equivalent of another refer.
I agree the Edge Star uses more elec than, say a Nat'l Luna. It also requires a larger footprint. However, I already own it. It's relatively new; and saves me start-up $ if I can power it.
So, until I build something and am able to audit actual numbers, I'm staying with an expected need of 120Ah.

Wow, the 12 volt O₂ Concentrators--Just looked them up--Roughly 5 amps @ 12 volts... That is not too bad.

Bill-
Thanks for the calls. Even if I add the bookend sunlight hours at some lesser %, and figure in a 25% boost from a MPPT, USING MY 2-160W glass panels may never replace the 150A I need if I use 50% of the rated 300.

Personally, I do not really use the MPPT is 1.2x more charging current. In general, you will only see a 10-15% improvement in sub freezing temperatures. At normal non-winter temperatures, I am not sure there is much of a measurable performance improvement (MPPT uses something like 5% of the power to run its electronics--PWM probably use 1% or so).

MPPT is great to connect "non 12 volt panels" to a 12 volt battery bank and/or use a "high voltage" solar array that is some 10's to 100's of feet from array to charge controller/battery bank. MPPT controllers cost more, but "GT Panels" (typically ~30 to 36 volt Vmp) are much cheaper than "12 volt panels)... So for larger system, MPPT can save some money.

Panels-
Let me try a different approach: I'm not sure glass panels will stand the rigors of Jeeping. So, I want to switch to flex panels. Renogy sells a 100W? While it doesn't roll up, it will bend and flex. At 42" x 22", I can lay 4 of them side-by-side on my Frontrumner rack. Would someone (Bill) please run the numbers and see what they can do toward re-filling my bank: Pm-100W x 4; Vmpp 17.7V,; Imp 5.7A; Voc 21.7V; Isc 6.1A.
A major advantage to me to go with these bendable panels is they only weigh 4.4 lbs. I will make some frames than can be easily put up and down on my rack allowing me to store the panels inside my Jeep. That isn't perfect security; but it's acceptable.

I am not a fan of flexible panels. My personal opinion (I am not in the business) is that flexible panels will last you ~5-10 years at best. Plastics just don't like UV/Hot Sun. In some environments, flexible panels may last 2 years or less (marine). I don't know Recology panels (or other specific brands)--If somebody has good luck with Recology flex panels--Let us know.

Recology panels seem to be pretty efficient, many flexible panels use thin film cells which are about 1/2 as efficient as crystalline panels--So check the size of the panels and see which will give you the most watts per square foot of space.

Glass panels, as long as they are not hit by rocks or subject to twisting/etc... They should work fine (i.e., these are just tempered window glass ~1/8" thick). The will hold up pretty well, but one good wack and they will shatter the whole face.

Controller-
My own calls, probably wrong, for 4 Renogy panels say I could end up seeing as many as 133V come down in good conditions, especially with a MPPT. Also, Amps could be up to 38. That doesn't give me enough margin to use a 40A controller. Looking at 50A MPPT's, there aren't many and they're all expensive. Now, my figuring was hooking all 4 in parallel. This is so one partially shaded panel won't draw down the rest. If I went series, instead and used 2 smaller controllers, I don't know if that would save anything.

In general, for MPPT type charge controllers, they seem to be most efficient around 2x battery charging voltage (i.e., charging at ~14.5 volts, run the array at ~29-30 volts). For a small system (short wire run from array to charge controller+battery bank) there is generally no reason to go to Vmp-array around 100 VDC (roughly the typical maximum Vmp-array for a high end MPPT solar charge controller will accept over a wide range of temperatures and operating conditions).

Shaded and partially shaded solar panels are bad news. You lose a lot of the output and, in some cases, can overheat the bypass diodes on the shaded panels (personally, I think bypass diodes in solar panels are a weak point in their design--Very difficult to get rid of waste heat from diode in plastic box/behind hot glass+cells).

If you are using Vmp~18 volt panels--You can put two in series for Vmp~36 volts on a 12 volt battery bank--And put them in parallel (2, 4, 6, etc. panell array).

If you are using Vmp~30 volt panels--Just put them in parallel for a 12 volt battery bank (for a 24 volt battery bank, 30 Volt Vmp panels in parallel is not "high enough" operating/charging voltage for a 24 volt battery bank).

-Bill