# Battery Systems 101

Solar Expert Posts: 63 ✭✭✭
I have been looking for some formulas and/or definitions to help me in battery sizing.
In my case i have 1 string of 8 each Trojan T-105 6V, 225 Ahr batteries.
1. What does this translate to in expected watts out.
2. What is the difference between getting 48 volts from 6V, 8V, or 12V batteries? Which is better?
3. If my load is 800 watts how long would the batteries last.
4. What is the formula for required panels to charge the above panels? Rules of thumb?
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

• Solar Expert Posts: 3,009 ✭✭✭✭
Re: Battery Systems 101

Your load of 800 watts, is this 800 watts just once in a while, or is it 800 watts 24/7 ??
What would be your total watt hours used per 24 hour day? If it's a 24/7 load of 800 watts, that works out to over 19,000 watt hours per 24 day, NOT INCLUDING all the standard and normal losses in the system. A huge load for a home solar system, which would likewise require a relatively huge system to feed.
• Banned Posts: 17,615 ✭✭✭
Re: Battery Systems 101
mnittler wrote: »
I have been looking for some formulas and/or definitions to help me in battery sizing.
In my case i have 1 string of 8 each Trojan T-105 6V, 225 Ahr batteries.
1. What does this translate to in expected watts out.

This amounts to 225 Amp hours @ 48 Volts. At a maximum DOD of 50% that's approximately 5.4kW hours DC. We say "approximately" because a number of factors such as rate of discharge, actual battery Voltage, and temperature will affect the actual amount of power available.
In AC power terms it is less because allowance has to be made for the inverter consumption and conversion efficiency. It is best not to draw the batteries lower than 25% average DOD.
2. What is the difference between getting 48 volts from 6V, 8V, or 12V batteries? Which is better?

Theoretically there is no difference. In practical terms there can be because of the number of connections made either in serial or parallel for equal stored capacity.
3. If my load is 800 watts how long would the batteries last.

800 Watts DC on the 225 Amp hour 48 Volt batteries is approximately 17 Amps. In theory that is approximately 6.5 hours for a continuous draw. As before this is not a precise number due to the mitigating factors. When the batteries are fully charged the Voltage will be higher and thus the current draw lower for the same Watts output. As the batteries discharge and the Voltage goes lower the current will increase for those same Watts, and the fall off can be quite dramatic.
4. What is the formula for required panels to charge the above panels? Rules of thumb?

The rule-of-thumb is to use 10% target peak charge current, or 22.5 Amps. If you were to use a PWM controller this can be converted to panel Watts by multiplying by the Vmp for a 48 Volt system: 70 * 22.5 = 1575 Watts. For an MPPT type controller you can do it this way: 22.5 * 48 / 0.77 = 1403 Watts (slight efficiency improvement with MPPT controller). Of course you can't buy panels in any size you want, so you should round the number up to fit an available configuration.

You should always check the array size against the intended power use in Watt hours using the equivalent good sun hours for your location and the over-all efficiency factor of 0.52: 1403 Watts * 4 hours * 0.52 = 2.9 kW hours. This should fit with a 25% DOD which in this case is 2.7 kW hours. If you discharge deeper you will need to recharge faster which means a need for more panel. The same goes for other interference with charging such as short hours or low insolation.

How's that?
• Solar Expert Posts: 63 ✭✭✭
Re: Battery Systems 101

Reference my 48vdc system:
A bunch of questions from a battery dummy:
I was looking thru the parameters of the Xantrex xw6048 last night and was wondering what the correct settings should be.
What should the charge voltage be (I am assuming maximum voltage to quit charging)?
What should sell voltage be (I assume to start exporting to the grid)?
What voltage should the Solar charge controller be set at? I am confused here because the Xantrex xw6048 can charge the batteries from the Grid and I will also have a MPPT Solar Charge controller so I do not want them to fight each other.
In my case I can bank (via export) up to 500 kw with the utility during the month and then use them before the end of the month. After 500 kw I have to export 2kw to get 1kw back. Am I right in thinking of this as a 500kw battery bank using the utility as a battery bank that gets discharged at the end of the month (monthly net metering)?
What should the cut off (inverter shut down) voltage be set at?
What does DOD mean?
To get better battery life what should the cutoff voltage be?
What does 85% DOD mean and what voltage would this be?
What does 50% DOD mean and what voltage would this be?
Thanks for the education.
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

• Banned Posts: 17,615 ✭✭✭
Re: Battery Systems 101
mnittler wrote: »
Reference my 48vdc system:
A bunch of questions from a battery dummy:
I was looking thru the parameters of the Xantrex xw6048 last night and was wondering what the correct settings should be.
What should the charge voltage be (I am assuming maximum voltage to quit charging)?
What should sell voltage be (I assume to start exporting to the grid)?
What voltage should the Solar charge controller be set at? I am confused here because the Xantrex xw6048 can charge the batteries from the Grid and I will also have a MPPT Solar Charge controller so I do not want them to fight each other.
In my case I can bank (via export) up to 500 kw with the utility during the month and then use them before the end of the month. After 500 kw I have to export 2kw to get 1kw back. Am I right in thinking of this as a 500kw battery bank using the utility as a battery bank that gets discharged at the end of the month (monthly net metering)?
What should the cut off (inverter shut down) voltage be set at?
What does DOD mean?
To get better battery life what should the cutoff voltage be?
What does 85% DOD mean and what voltage would this be?
What does 50% DOD mean and what voltage would this be?
Thanks for the education.

Okay first problem: you have 225 Amp hours of battery, right? For use with an XW6048 and grid-tie/sell that needs to be 3X that size as per Xantrex recommendations of 100 Amp hours per kW of inverter. There's quite a few discussions of this around the forum and it all has to do with the batteries filtering out the ripple current. Until you fix that I'd recommend avoiding selling to grid.

The charging Voltage is as per Trojan recommendations i.e. Absorb at 59.2 and use a remote temperature sensor.

The inverter Low Voltage Disconnect is best set at the nominal system Voltage, in this case 48. That represents a resting Voltage of 50% SOC so when the load is remove it should 'rebound' above that point. Kind of ensures the batteries are never taken down below 50%.

DOD is Depth Of Discharge, the reciprocal of SOC which is State Of Charge. 85% DOD is a dead battery as it is only 15% SOC. 50% is the same for each.
• Solar Expert Posts: 63 ✭✭✭
Re: Battery Systems 101

Well, I ran a test with the single string of T-105's running the house critical loads with the Xantrex XW6048 grid (AC1) turned off with no solar available. I have a 48V battery system. The Low voltage cutoff was set at 46V and generator start was set at 47.5V and after 5 hours supplying the critical loads at night the generator started and began charging the batteries automatically. Anyway, that test seemed to work (generator started). I have the charge rate set at 22% (22 amps) amps.
My question is what should the parameters be set at for optimum performance considering 99.9% of the time the Grid will be available and not harm the batteries.
Me not knowing anything but reading the manual and you having experience what would you do?
I am currently in the process of adding the 2nd T-105 battery string and the 1.8kw solar (3 strings of 2 each 300 watt panels) and Xantrex XW-MPPT60-150 charge controller connected to the batteries.
What would you put for parameters? The () is what I have now.
Low Voltage Cutoff (46v) I have no idea if this is the correct value.
High voltage Cutoff (70V) Must have been a default since I did not enter it.
Charge Cycle (2StgNofloat) Default
Recharge Volts (51.0) I think this was 50 and I changed it to 51.0 for no reason but to see what happened)
Grid Support (I think it is off)
Generator Start Voltage 47.5V 15 sec
MPPT Charge Controller voltage ?
And whatever other parameters you know I need to change from Defaults?

I think my goal would be to have the batteries available if the grid fails for almost a whole house UPS critical loads panel (some of the 120vac circuits and none of the 240vac circuits) use.
The charge controller to charge the batteries every day and sell excess after batteries are topped off. (I also have grid tied Solar and Wind equipment connected to the Grid to assist as long as the Grid is present). I plan on adding a 3rd battery set as funds allow. I also have net metering up to a maximum of 500kw per month with my electricity provider and 1kw back for every 2kw I give them after 500kw.
Have XW6048 keep the batteries somewhat charged? and the charge controller charge them after that during the day. I also plan on adding another 1200 solar watts to the charge controller as funds allow.
So maybe use some of the stored battery energy during the night but not enough to harm the batteries?
As you can tell I am very new at this and the complication of having grid tied equipment and the Xantrex xw6048 adds to the confusion mix. So many parameters, what is a fellow to do. (I can't even spell most of them much less know what they do?) I want to try and not kill the \$1100 per string batteries since I know others have already done that and I want to learn from their mistakes not mine?
Any help will be greatly appreciated.
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

• Solar Expert Posts: 3,741 ✭✭✭✭
Re: Battery Systems 101
mnittler wrote: »
Well, I ran a test with the single string of T-105's
<snip>
I have the charge rate set at 22% (22 amps) amps.

22 amps is a 10% charge rate on a single string of T-105s.
mnittler wrote: »
My question is what should the parameters be set at for optimum performance considering 99.9% of the time the Grid will be available and not harm the batteries.

Good question. Those batteries are not optimized to live their lives in float. It would probably be a good idea to exercise and equalize them regularly.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Banned Posts: 17,615 ✭✭✭
Re: Battery Systems 101

There aren't many things I wound change other than the LVD: set to 48 Volts. At that level under load you can be sure the batteries are above 50% SOC. The solar charge controller should have the same charging parameters as the XW.

Another issue you have is only 220 Amp hours on a 6kW inverter ... well if you really loaded up that inverter you'd see dead batteries in a hurry. And if you wanted to grid tie it (since it has the ability you should probably consider it) you'd want at least 600 Amp hours capacity. Adding battery sets "as funds allow" can be problematic, but as these really won't be cycled much if you can keep an eye on them you may have up to two years to connect another string.

Your 1800 Watts of panels is fine for one set of batteries (1800 * 0.77 /48 = 28 Amps), minimal for two (6% rate) but won't keep up with three (3% rate). You will be relying on grid to charge the batteries if used and the panels to maintain them when they aren't. Of course if you do go for selling to the grid the array would need to be increased to where that is practical: no sense putting 1.8kW through a 6kW inverter.

There are several forum members who have XW6048 inverters both on and off grid who may have further insight.
• Solar Expert Posts: 63 ✭✭✭
Re: Battery Systems 101

I moved the Generator auto start up to 48V (after 15 Seconds) and the low voltage cutoff to 47V. Will this do the same thing in protecting the batteries or should I have the Generator auto start at 49V and the low voltage cutoff be at 48V as you indicated. I also changed the XW6048 charger to 52.0V recharge volts (should this be 52.8V as per Trojan literature?). I also have 10kw of grid tied solar connected to the system so there is and extra amount available from the grid during the day which is connected to the Xantrex AC1 connection as long as the grid is present. I should have the 2nd T-105 battery string connected within the next 2 weeks. I experimented with changing the Xantrex charge volts to 52.8 and saw the batteries charging (7.0 amps down to 1.7 amps then
display) for a little bit then I saw the battery voltage go to around 58V for a short while then settled back to around 53V after 10 minutes or so. Is this normal? I still have the Xantrex charge setting 2Stage No Float. The 1.8kw Solar with the MPPT charge controller is not yet connected.
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

Re: Battery Systems 101

Monitor you bank state of charge and see how much generator run time you get (and how much sun).

Making ~46 volts @ 15 second and 48 volts @ 15 minutes would seem to me, a good compromise for reducing generator run time and fuel usage vs battery life.

If you have very heavy surge loads (well pump) and short time heavy loads (microwave, etc.)--I would probably want to configure the system to not start the generator for those events.

What you are looking for is when the battery voltage/capacity is low, the moderate loads are going to run for a while (well pump irrigation, clothes washer/dryer, bunch of warm food in the fridge, etc.) and not much sun--You want to genset to start and run for 15+ minutes to get it warmed up and water/by products vaporized out of the oil.

Continuing to run until >80% SOC (assuming bad weather/heavy night time loads) is going to be fuel efficient (heavy charging current, batteries not gassing).

Each system/usage is different, and you may have to reprogram based on battery temperature (I am not sure how many, if any, generator controllers are temperature compensated for battery voltages).

By monitoring your bank/system operation, you will come up with set of operating conditions that work well for you.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭✭
Re: Battery Systems 101

I don't know what is "normal" for the operation of an XW. To me they work strangely, and are known to be buggy. So the Voltage display may be normal or it may be a sign of yet another XW flaw. I would guess it is related to the "No Float" setting and that the lines appear when it should switch to Float but since this has been disabled you get a cut off (----) followed by the charger shutting down and monitoring of battery Voltage (53V) until it reaches recharge Voltage and then the charging will begin again. If I were you I'd set the Float to ON and give it a reasonable value such as 55V. Then see what happens.

Your AC only GTI isn't going to make any difference as to the performance of the XW because it is just another AC source paralleled to the grid on AC1. The XW doesn't care that some of its power there comes from solar and some from a generating plant.

The autogen start at 48 Volts and LVD at 47 should work fine. Since this is under load even 47 Volts is probably not below 50%. That way the gen should start at 48 and if for some reason it doesn't the whole thing will shut down at 47. I believe there's a reconnect Voltage setting as well. That of course should be set at a level where operation could be held steady, such as the recharge point of 52V. I wouldn't worry about the difference between 52 and 52.8 on that; Trojan is probably looking at it from a usage POV where the number represents a minimum Float value.
• Solar Expert Posts: 63 ✭✭✭
Re: Battery Systems 101

I took your advice and decided to install the 2nd battery bank to the XW6048 now instead of later so it is now installed and charged up (16 each Trojan T-105's). This way the batteries will all be basically the same age. The 3rd battery set will probably not happen in the near future unless I find out that I really need them to operate. This xw6048 system is used 99.9% of the time on grid and off grid only if the grid is down. I have a separate critical loads panel tied to the XW6048 AC Out that is only powering some of the 120vac house circuits, (TV, DVR, Internet router, solar water heater very small circulating pumps, refrigerator, freezer, video surveilance system, Alarm system, some overhead lighting, power monitoring system, and some wall plugs. I tried to balance out both sides of the panel as best as possible. Most of the overhead recessed lighting has been changed out to 13 watt LED and the remainder is CFL. I have been watching this critical loads panel and it seems to max out at around 1800 watts but usually hangs around 400-800 watts (with a low of 150-250 watts) if we are not being conservative. I experimented and made a pot of coffee with AC1 turned off the other day with the electric drip coffee maker and the inverter held up just fine. None of the 240V circuits (water pumps, clothes dryer, stove, pool pumps, air conditioner, heaters, etc.) are on the inverter. I figured that if I need the 240vac circuits or wanted to make coffee then I would probably just start the generator if the grid was down. My logic is that my other grid tie Solar and Wind inverters or generator will supply power to charge the batteries as required. I am intending on installing a 3.0kw solar battery charging system within the next 30 days for the battery banks.
My question is on the Xantrex XW6048 settings. (It is amazing to me how complicated and complex it is to charge batteries and have them function and survive).
I have now set the parameters to the following: Is this what you would set them at?
XW6048 Charge voltage = 50.2V (2 stage with no Float) (Is this high enough only entered since I read 50.2 is 100% SOC)
Charge amps = 45 amps
Absorb charging = 120 minutes (I watched this thru one absorb cycle and it was still around 8 amps charging when the 120 minutes expired) Should this be increased?
Grid Support voltage = 53.0
Grid support = On (I have never seen this do anything)
Sell = No (I would like to sell rather than shut down the charging circuit when the 3.0 kw solar charging panels are installed in order to help with the non critical load panel loads)
Generator auto start voltage = 48.0V @ 15 seconds
XW6048 Inverter shutdown voltage = 47.0V @ 90 seconds
When I install the 3.0kw battery charging panels what charge voltage would I set the Xantrex MPPT60-150 to?
Will the Grid Support voltage of 53.0V supply power (assist) to the critical loads panel when the voltage is over 53.0V. (Which currently cannot happen?)
I am just trying to figure out how to make this thing dance. I also have 5.1kw and 5.4 kw grid tied Solar inverters and a 10kw grid tied wind turbine connected that function pretty well as long as the grid is present. I guess I am thinking of this XW6048 as a partial whole house Smart UPS for critical loads.
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

• Solar Expert Posts: 63 ✭✭✭
Re: Battery Systems 101

Would it be possible to connect strings of batteries with different Ahrs but the same string voltage?
Example the following strings connected in parallel?
8 each T-105 6V 225 AHr batteries (48vdc)
6 each T-875 8V 170 Ahr Batteries (48vdc)
4 each T1275 12V 150 Ahr (48vdc)
Can this be done?
If not, Why not and what would happen?
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

• Banned Posts: 17,615 ✭✭✭
Re: Battery Systems 101
mnittler wrote: »
Would it be possible to connect strings of batteries with different Ahrs but the same string voltage?
Example the following strings connected in parallel?
8 each T-105 6V 225 AHr batteries (48vdc)
6 each T-875 8V 170 Ahr Batteries (48vdc)
4 each T1275 12V 150 Ahr (48vdc)
Can this be done?
If not, Why not and what would happen?

Not a good idea.
The reason being different battery capacity have different charging/discharging profiles.
Let's add them up and see what happens:
225 + 170 + 150 = 545 Amp hours.
Now let's take 10% of that and apply it:
54.5 Amps unevenly divided by three battery strings with different resistances throughout their SOC.
The lower capacity battery will have the least resistance and thus receive the most current whereas the highest capacity battery will have the most resistance and therefor receive the least current. Exactly the opposite of what is needed.
Even if the current could be applied as 1/3 to each string (18 Amps) the result would be:
8% charge rate for the 225 Amp hour batteries, 10% charge rate for the 170 Amp hour batteries, and 12% charge rate for the 150 Amp hour batteries. (Some mathematician can work out what the ratios actually would be. Possibly 29%/35%/36% meaning the 225's would get 15.8 Amps or 7% rate, the 170's 19 Amps or 11% rate, and the 150's 19.6 Amps or 13% rate. But I'm just guessing at the distribution.)

There is simply no way that all three strings will reach the same SOC in the same amount of time because they are all being charged at different rates. The same applies for discharging, but the two do not 'balance out'.

Even left on their own the higher capacity batteries will force current into the lower ones, and over time will lose capacity. This is the same reason why it is not a good idea to mix 'identical' batteries of different ages; the older they are, the more used and/or abused, the less real capacity. The greater the difference between any two the worse the effect.
• Solar Expert Posts: 722 ✭✭✭
Re: Battery Systems 101

A friend tried that with his 12v system, had (4) T145's and added (2) T105's and at one point the T105's went into a thermal runaway state...so badly that the temp monitor on the FM80 actually said 'Battery temp Too High" and you could barely touch them with your bare hands.

Might have been other factors, but we guessed it was because of the unbalance.
Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
• Solar Expert Posts: 63 ✭✭✭
Re: Battery Systems 101

I have now added the 3rd 48v T-105 battery string to the controller. I have added one string per month so there is a 3 month difference between the 1st string and the 3rd string.

I had used a 12v inverter (to have a work light, power the wife's laptop, have some house light using a lamp, and power a TV) across 2 of the 6V batteries (to make 12v) in one of the 48v battery strings while the main inverter was down and while we were doing some house critical load re-wiring that took almost 6 hours. I moved the 12v inverter to another set of 6V batteries twice during the day. After the string had being connected to the Xantrex XW6048 inverter/charger for nearly a week those 4 batteries that I had been used with the 12v inverter continued to measure lower voltage than the rest of the batteries in the string. That was amazing to me and solidified what some of you had been saying about the importance of having all the batteries the same. I took that string off line and charged the 4 each lower voltage batteries with 2 each 12v trickle float charger (Battery MINDer from my boat) until they came up to the same voltage as the rest. It took almost all day. I then connected the 2nd 48v bank back to the Xantrex system and other string for about a week. I tested the SpGr and voltage of the string and now all batteries in all the strings are nearly identical. This made a believer out of me that there is more to battery science than just a hunk of lead, battery acid and a plastic case. I would have never thought that the batteries would not recover on their own. I also put the entire system thru a equalize cycle just to balance out all the strings after adding the 3rd string.

I left the new string on charge for 24 hours before connecting it in parallel to the other strings and there was no spark when I connected the 3rd string with no load. I don't understand it, do not know why, but it is true about battery strings being very finicky.

After I tested my renewable energy batteries i went over and tested my Golf Cart string and lo and behold the 2 6V batteries that were connected to the Golf Cart lights (12v lights that are used very seldom) were lower voltage than the rest. I have now also bought a 48V to 12V 30 amp voltage reducer for my Golf Cart lights. My previous logic was 12v was 12v but that as far from the truth as possible when dealing with battery strings. Previously it was hard for me to justify spending \$100 on a 48V to 12v voltage reducer when all I had to do was connect across 2 of the batteries in the bank. I figure this voltage reducer will probably save me the cost of 2 batteries that would have gone bad before their time and probably the cost of replacing the entire battery string. I charged and balanced out the Golf Cart battery 48v string and will see what happens in the future.

Thanks for the education.
19.76kw Solar/GT Enphase IQ7+ MicroInverters
5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

• Solar Expert Posts: 722 ✭✭✭
Re: Battery Systems 101
mnittler wrote: »
Previously it was hard for me to justify spending \$100 on a 48V to 12v voltage reducer when all I had to do was connect across 2 of the batteries in the bank.Thanks for the education.

For 48v to 12v, this works great for me to run my 12v sureflo pump.
http://www.ebay.com/itm/161253580556?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Good up to 10amps at 12v and only \$30

I know the host sells one for a LOT more \$. Not sure the difference.
And I love the host and have spent thousands of \$ with them. So, sorry to post a link to another product but it seemed to fit the need.
Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html