How to size dual battery supply ckt?

[bicyclist]

I need to know how much current will be drawn by, say, a 220AH pair of 6V FLA batteries at 50% discharge (or, what the heck, 100% discharge) so I know how to size the supply circuit. It'll be a house battery in a Transit with a 220A alternator. Ford wants you to hook up to their "customer connection point", which consists of 3 terminals, each with a 60A fuse.

The battery will be charged by a pair of 160W solar panels, but if there should come a dark time, I want to be able to use the alternator as a backup.

Thanks for any advice.

 

[DiploStrat]

Use a voltage drop calculator. You will need to know the charging voltage of your truck's alternator and you can probably round 200A down to 175-200A. Ideally, you would like the voltage drop at full output to be about 0.5v or less.

N.B. The batteries will draw more at 50% SOC than at 0% SOC or 100% SOC. In any case, they can't draw more than your alternator produces.

Try this: http://www.calculator.net/voltage-d...nce=15&distanceunit=feet&amperes=200&x=26&y=8

 

[Vanaroo]

I need to know how much current will be drawn by, say, a 220AH pair of 6V FLA batteries at 50% discharge (or, what the heck, 100% discharge) so I know how to size the supply circuit. It'll be a house battery in a Transit with a 220A alternator. Ford wants you to hook up to their "customer connection point", which consists of 3 terminals, each with a 60A fuse.

The battery will be charged by a pair of 160W solar panels, but if there should come a dark time, I want to be able to use the alternator as a backup.

Thanks for any advice.

When I size that sort of wire, I sort of "circle around" through the various requirements until I arrive at the correct wire size. One is how much current (amps) can my charging sources output, and the other is how much voltage drop do I want (or not want, really) (I don't care whether my battery bank is 50 amp hours or 500 amp hours -- I only need to know about charging sources and loads.)

I don't completely understand a 220 amp alternator going through a 60 amp fuse, but I'm not familiar with the Transit. I can give an example of how I sized things on my E-350; maybe reading through my thought process will help you.

So, I have a start battery in the Ford, and IIRC around a 120 amp alternator (I knew specifically what it was when I was doing the project though!). I have an 18 foot one-way run from the start battery to my house bank. My house bank is three Group 31 AGM's, for 375 amp hours (but none of this is related to the size of the house batteries - the amperage flowing is determined by the charge sources and the loads).

I not only want to be able to charge the house bank, but I also want to be able to "self jump start" if needed. So that adds other parameters. The starter on my engine draws around 150 amps (inrush current doesn't count).

So, I need to be able to fuse up to around 200 amps to accommodate the starter draw. Hence I immediately know I need wire with an ampacity of 200 amps in an engine space. I look on the ampacity table for 105ºC rated wire not bundled (this will be run singly) (I use marine wire by Ancor) and see that anything 1AWG or larger will work. Actually you are allowed some leeway in fusing but since voltage drop will probably make me want to run larger wire anyway, I move on along in my calcs.

Now I want to see about voltage drop when I'm charging my house bank. I go to the calculator and put in my specs (round trip wire length, wire size, amperage, temp, etc.) and see that with 1AWG wire, and presuming my alternator is putting out around 60 amps (I don't typically see more than that heading to the house bank - presumably the engine etc. use the rest). I get a voltage drop percentage of 2.8%. That's okay. I would never accept that for a solar input, as the solar controller is "smart" and can deliver a very precise voltage; but the alternator is just "stupid" bulk charging, basically.

Now I look to see what the voltage drop would be in the other direction, while starting. That would be 6.9%. Not great, but I don't plan on self jump starting often or for long.

But then I don't have any 1AWG wire. I have 2AWG (smaller) or 1/0AWG (larger). So I go with the 1/0.

A friend with the same exact rig elected to go with 2AWG for the same thing. He was able to "fudge" slightly on the fuse size (175 amp blew, 200 amp is fine) (you are allowed to "over fuse" slightly), and he didn't care too much about voltage drop. He has an ample solar system and only uses the start battery connection for occasional bulk charging.

I'm basically in the same position, but I use my start battery charging a little more often, so I wanted less voltage drop, plus I had the wire on hand.

None of the above really had anything to do with the size of my house bank, but rather the amount of current the charging sources could put in, or the loads take out, plus proper fusing (or breakering, as the case may be).

So, questions for you: What is your overall charging plan for the house bank? Is the alternator a primary source or a secondary one? How else do you charge your house bank? What's your usage pattern?

I don't really remember how fussy FLA batteries are about charging voltages. I have AGM and strive to give them just what they want (temperature compensated). If I were using my alternator for a primary charge source I'd be looking into a "battery to battery" charger which could be adjusted.

But even if your house bank is not fussy about voltage, you still will want to be charging at over 13 volts, so won't want too much voltage drop. Plus you will need to be able to fuse both ends of the wire for safety, so need wire that can take the correct fuse size (ampacity).

I'm interested in the 60 amp Transit factor (might want one myself someday).

 

[bicyclist]

N.B. The batteries will draw more at 50% SOC than at 0% SOC or 100% SOC.

And that's what I want to know. The batteries offer some resistance. How much?

 

[bicyclist]

I'm interested in the 60 amp Transit factor (might want one myself someday).

OK, I'll start here. The Transit can be ordered with a 220A alternator and a pair of AGM batteries which sit under the seat. Mounted on the side of the seat is the "customer connection point" with 3 terminals, each with a 60A fuse. The CCP is supposed to be used by upfitters to power whatever. You can also order a set of 4 upfitter switches, each of which is fused at 20A. Ford discourages you from connecting directly to the battery, though I don't know why.

My intention is to use the solar panels as the primary charging source and to use the alternator as secondary, so I don't really care about having the proper charge profile. It'll simply provide some bulk charging. I don't intend to ever jump the start battery from the house. The house battery will be located behind the seat 2 or 3 feet from the start battery, so negligible voltage drop. Now those 3 60A CCP fuses are buried and I don't want to ever have to change one. I'm trying to figure out whether I can connect to one or more of the CCP points to charge the house battery without blowing a 60A fuse.

 

[Vanaroo]

Okay, now I understand better. Thanks for the detail.

So, if you are not planning to jump start, you immediately have no need to account for the big amperage pull of the starter. Thus you have the amps out from the alternator going "back" to the house, and that's all you really need to account for. It sounds as though your system will be very similar to mine in that I charge by solar 99% of the time and only use the alternator occasionally when I need a bulk charge. In actual use this ends up being less than a day per month for me. Side note is that because I use it so infrequently (and because I *don't* want to have them connected otherwise), I fit a manual switch instead of an automatic charging relay. I just leave it off (not combined) 99% of the time, and switch it on for that 1% of the time. I didn't want the alternator charging my house bank when I didn't want it to; and I don't have any noticeable parasitic loads on my start bank so I didn't want the solar charging going that way (unless I choose it). I used a Blue Sea switch, and then wired up two LED's that both come on when the circuit is combined. A solid green light at the dash, and a flashing red light in the "house" (so I can't forget and leave them combined). (I have had an automatic relay on my boat, so nothing against them in principle.)

As to your batteries "demanding" huge amounts of amps -- I see your point. Normally the batteries won't take more than the charging source can "give." But now you have a 220 amp alternator and only a 60 amp fuse. So will that cause a problem if your bank is super low and "hungry." My guess is maybe not, but that's pretty weak because I'm not sure. FLA don't accept charge as fast as some other types, so you have that in your favor. And you will hopefully not be going below 50% too often (unless your plan is to carry less lead and replace it more often, which can be viable). So yeah... if they were 110 ah down, would they try to take more than 60 amps. Good question. I've never had to connect to something that was fused so much lower than the charging output (220 amps vs. 60). Be interested to see what others have to say about this.

On wire size choice:
If you have say.... a 20' one-way run and run 2AWG, you'd be at around 3.9% voltage drop at 12 volts, 3.3% at 14 volts. 2AWG 105ºC rated wire, not bundled, and running in an engine compartment can be fused to 178 amps, so you are beyond fine there. If you wanted to go up to 1/0 then at 60 amps you'd have 2.4%/2.1%. OTOH, if you dropped down to 4AWG, you'd be at 6.2%/5.3%. If the alternator is putting out 13.6 volts (just to pick out of a hat), then with 5% voltage drop your house batteries would be seeing 12.9 volts. Generally I'd like to see around 13.3 for float on my batteries and so bulk would be between that and their absorb voltage of 14.4 (at 77ºF).

So if it were me with your run, I'd probably go with 2AWG and use an MRBF right on the positive post of the house bank to fuse the other end of that wire, providing you have the height. I don't know what the short circuit rating of your batteries is, but with the AGM's I have 220 amp hours worth of them would have a short circuit rating of just over 7,000 amps, so the MRBF's AIC rating would be fine at 10,000 amps.

(AIC is ampere interrupt capacity and is basically a rating of the body of the fuse. Battery banks can let loose a large amount of energy all at once if there is a short, and the fuse doesn't do any good if the fuse holder or body blows apart, melts, or etc. Hence the AIC rating of various fuses/holders.)

Then you'd likely want to put a switch or an ACR (such as the Blue Sea 7622) in the combining circuit. For a switch I have used both the Blue Sea 9004e and the Blue Sea 6010. From the "extra" terminals on either of these is where I get power for the LED indicator lights I use.

I hope someone else has some input on the 220/60 question (which I realize was your main question!)

 

[bicyclist]

Okay, now I understand better. Thanks for the detail.

So, if you are not planning to jump start, you immediately have no need to account for the big amperage pull of the starter. Thus you have the amps out from the alternator going "back" to the house, and that's all you really need to account for.

That's correct

It sounds as though your system will be very similar to mine in that I charge by solar 99% of the time and only use the alternator occasionally when I need a bulk charge.

Yessir.

On wire size choice:

I have 2AWG on hand and the distance between start and house is maybe 2 or 3 feet, so no problem there.

So if it were me with your run, I'd probably go with 2AWG and use an MRBF right on the positive post of the house bank to fuse the other end of that wire, providing you have the height.

Agreed.

Then you'd likely want to put a switch or an ACR (such as the Blue Sea 7622) in the combining circuit.

Right, I want a manually controlled isolator of some kind so that it is on only when I want it to be. I use a manually switched solenoid in my Tacoma.

I hope someone else has some input on the 220/60 question (which I realize was your main question!)

Me too, and thanks for your thoughts.

 

[Vanaroo]

Well with a 2-3' run you can use dental floss I wish I had such a short run!

Also sorry that it looks like I basically re-stated what you already knew, and didn't answer your question. Geez.

Have you by any chance asked on the Ford Transit USA forum? Not to send you away but it sounds like one of those things that the people who are all up on all the various option packages might be able to shed some light on - especially as pertains to hooking up more directly to the alternator or battery (vs. the 60 amp fuse). Or maybe you already have checked...

On the switch: My buddy's Blue Sea switch can be reached from either the driver's seat or from in the back, so it's just peachy. Where I put mine I can't reach it from the driver's seat, so I'm thinking of either moving it, or perhaps using a Blue Sea remote switch (form factor like a 7622 ACR, but actually just a switch with additional remote switching capability). Maybe it doesn't really matter, but for those times when I do combine, I prefer to start with them not combined and then combine after running. Of course by then half the time I'm already driving.... hence my plan to either move it or go with the remotely activatable one. Likely you are already all set on this though (but maybe someone else reading will be interested).

 

[4x4junkie]

I'm not familiar with the Transit and that CCP thing so this is only a shot in the dark, but I imagine all three of those 60A outputs must be coming directly off the same power buss, no? If so, I don't see why you couldn't just wire all three in parallel to produce a single 180A output which I suspect would have no issues charging up an aux battery bank no matter how much current it wants. I have found vehicle alternators pretty consistently fall about 15-25% short of whatever their max ratings are, so it's possible you might be completely fine @ 180A.

Me, I'd just tie onto the main battery (unless Ford actually has some dumb legalese in the warranty about that).

 

[bicyclist]

Have you by any chance asked on the Ford Transit USA forum?

Yeah, a number of folks on that forum have installed a variety of systems and it's obvious that some of them are pretty sketchy, thus the question over here where there are some more knowledgeable people. So, it's read a lot, ask some questions, and see what makes sense. I don't know enough about charging batteries, but I'm learning.

Maybe it doesn't really matter, but for those times when I do combine, I prefer to start with them not combined and then combine after running.

That's the way I do it in my Tacoma. Makes sense to have the switch up front.

 

[bicyclist]

I'm not familiar with the Transit and that CCP thing so this is only a shot in the dark, but I imagine all three of those 60A outputs must be coming directly off the same power buss, no?

Yes, that's correct.

If so, I don't see why you couldn't just wire all three in parallel to produce a single 180A output which I suspect would have no issues charging up an aux battery bank no matter how much current it wants. I have found vehicle alternators pretty consistently fall about 15-25% short of whatever their max ratings are, so it's possible you might be completely fine @ 180A.

I could do that. If I can get away with not using all three, it leaves the others available for other uses. I could experiment and hope not to blow a 60A fuse. The problem is that you have to remove the seat and pull out the batteries to gain access to the fuse. That's a PITA I can do without.

Me, I'd just tie onto the main battery (unless Ford actually has some dumb legalese in the warranty about that).

In their Body and Equipment Mounting Manual (BEMM), they specifically tell you to use the CCP. Can't remember where I read that they don't want you to connect directly to the battery. I don't quite see the difference between connecting to the battery or a point that is connected to the battery. Could be that they want to guarantee that there's a fuse in the circuit or might have something to do with their smart charging system. Dunno, but I'd prefer to do it the "correct" way. Several Transit owners have wired directly to the battery and haven't reported any disasters, yet. One guy didn't fuse the lead from the battery. That's an example of "sketchy" that I referred to above. He later added a 60A fuse between his house battery and his CTEK system. He hasn't blown the fuse, so the battery is charging at less than 60A. In my case, I don't think it's necessary to install a DC to DC charger just to do occasional bulk charging, but I'd like to know how much current the battery will draw.

 

[Vanaroo]

He hasn't blown the fuse, so the battery is charging at less than 60A. In my case, I don't think it's necessary to install a DC to DC charger just to do occasional bulk charging, but I'd like to know how much current the battery will draw.

I'm just thinking out loud here, because - as mentioned above - I don't know for sure. But since no-one else has said for certain yet...

I have AGM batteries. This is for a number of reasons that make sense for my rig, but the reason I mention it is that of course I've read up on them, and one of the things that is a (potential) advantage of AGM's is that they have a high charge acceptance rate (CAR). That is, they will accept (maybe you could say "gobble up") a huge amount of charge. This can be a plus if, say, you have a generator that can deliver lots of amps, and want to quickly bulk charge (boat scenario). Figures I have read say that the CAR for AGM's is around 30% to 50% of their capacity. So let's say I had a 220 amp hour bank of AGM batts and I discharged them to 25% (the horror! ). That would mean they wanted 165 amp hours put back. 50% of that would be around 82 amps. Maybe your 60 amp fuse would blow....

But you have FLA batteries. They are said to have a CAR of around 25%. So that same 220 ah battery bank, needing 165 amp hours, could only accept (or grab, as it were) 41 amps. The 60 amp fuse would not blow.

If your plan is to maximize battery longevity in terms of time, you will only be discharging to around 50%. That means 110 amp hours would need to be put back. 25% of that is only around 28 amps.... (Of course another philosophy is to carry around less weight in lead, discharge further, and buy fewer batteries more often, in which case you might be regularly discharging more deeply, but you could do the math for your scenario with the CAR and your planned depth of discharge.)

I doubt those percentage figures are exact, but I believe the concept is correct and likely they are reasonable rough figures. In which case, you'd likely be fine with the 60 amp fuse.

Just my natterings as I don't know for sure. I did double check with this article, written by a well respected marine consultant, because while I knew AGM's had a higher CAR than FLA, I didn't remember percentage figures. It's from 2011, so already slightly a period piece in some ways

http://www.passagemaker.com/channels/new-battery-technology/

Vanaroo

 

[bicyclist]

Ahah! Now we're making progress. I'll follow up on that. Thanks for the info.

 

[DiploStrat]

And that's what I want to know. The batteries offer some resistance. How much?

The battery cannot pull more than your alternator can produce. Ideally, you want to size your wire for that. If your really only have a 1 meter run you are very lucky. Charge rate is a function of the voltage difference between the two batteries; as the second battery begins to charge, its voltage will rise and the amp rate will drop. This is especially true of lead acid batteries and is the big reason that solar charging can be so useful, even it the maximum charge rate is low. Sadly, even with the best wire and the biggest alternator in the world, you are unlikely to see full output for even 30 minutes.

Undersized cable will still charge your second battery, but the amp flow may be so low that that is has no practical value. Run the numbers on 20 feet of 10 AWG wire and you will get the idea.

If you are sure that your Transit produces the proper voltage for your camper battery, I would simply use a intelligent charge relay and the appropriate wire - if you don't want to do the maths, use the same size of wire that Ford used to connect your starter battery. If the alternator voltage is too low, then you may be in the market for a B2B (battery to battery charger, or DC-DC charger) from CTEK, Sterling Power, or others.

 

[bicyclist]

Found some info that may be of use to other Transit owners.

https://www.fleet.ford.com/TRUCKBBAS/non-html/Q-226R1.pdf