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Ran D. St. Clair · 01-16-2010, 10:14 PM

400 Pounds of Electrons

I now have 6 each 6V batteries, for a total of about 400 pounds. These are top of the line AGM (Absorbent Glass Mat) batteries from Lifeline, GPM-4CT, for a total of about $1,500 with shipping and tax. They had to put them on a pallet and ship them freight but I am starting to get used to that where this project is concerned.

This means I have now made my last big ticket purchase. With the exception of the catalytic heater, which I had to send back because I ordered the wrong one, I have all the big ticket items in my possession now. My house is full of stuff that doesn’t belong, like a toilet in the kitchen, and a range in the living room. I am really hoping that my monthly cash flow will turn positive now. Unfortunately, as Blizz will attest, you can spend lots of money on the little things as well.

I went on line with McMaster Carr and found most of what I needed for battery cables pre-made.

http://www.mcmaster.com/#battery-terminals/=590gck

It’s not cheap, but it’s probably a lot easier than making your own. They have cables for paralleling up to 3 batteries, which is what I needed, and they also have cables for connecting the batteries in series. It’s handy that these cables seem to come in just the right lengths which is probably because at 66 pounds each, the batteries I used are about the same size as heavy duty truck starter batteries, so they connect together quite similarly.

The lugs on these pre-made cables are cast right onto the wire out of lead. Lead is fairly soft but the lugs are so thick that they seem to be plenty strong. The pliability of the lead probably also insures that it conforms to the mating surface with plenty of contact area. I haven’t noticed any heating even with high currents running through them, so they must be working well enough.

I did buy some 2/0 welding cable and solder on connectors for the link from the battery bay to the Inverter bay. I found them here:

http://www.mcmaster.com/#catalog/115/781/=590ep3

My experience with the solder on connectors is quite good so far. They were easily soldered on with a propane torch. Just strip the wire, push it into the connector, and heat the connector until the internal solder slug melts. Gentle pressure on the wire will force it into the molten solder and flux making a very strong joint. The solder did not wick up the wire much, so it remained flexible right up to the lug. Depending on the orientation of the lug, some excess solder may drip out, so put something down to catch it. Since the solder joint is up inside the lug, there is no way to inspect it visually. I did a bit of a pull and wiggle test and it seemed very solid though. I also found no heating at the lug even with very large currents running though it, so the implication is that it is a good electrical joint.

These solder on connectors aren’t cheap, but for just 4 lugs like this project, they are a lot cheaper than buying a large crimping tool for use with the crimp on lugs. If you already have a crimping tool, or can borrow one, the crimped lugs are probably cheaper.

The battery tray I am using is just 1/8” steel, about 22” by 22” and 1.5” deep. It fits the 6 batteries perfectly, which is dumb luck because it came with the truck as a slide in tool shelf. I removed the slides as I don’t think they are strong enough to carry the weight. I also put 6 lag bolts up through the top of the battery box into the wooden floor boards of the living quarters. The battery box is already welded to the steel floor joists, but at 400 pounds I wanted some insurance.

I thought long and hard about how to mount the batteries. I wanted them to be somewhat shock mounted to reduce the pounding. I was considering rubber vibration dampers, like motor mounts, but in the end I decided to keep it simple. I set the tray full of batteries on top of one inch of EPP (Expanded Polypropylene) foam with more of the same along the back, sides, and front, set within the door frame. I welded some D rings into the side walls of the box for heavy nylon straps across the top and another across the front to make sure they stay in the battery box.

I wanted to have some sort of a fuse or circuit breaker for the batteries, but I calculate the maximum required current from the batteries at close to 250A. I didn’t find any circuit breakers or fuses in that range at the local home improvement store. They might be available somewhere, but I wanted this design to be serviceable using readily available parts when possible.

High current circuit breakers can be very expensive. I also think it is dangerous to use a circuit breaker that was designed for AC in a DC circuit. An AC circuit has a zero crossing, or a point in time when the voltage is zero, every 8.3 milliseconds, which is an opportunity for any arc caused by the opening of a switch to extinguish. DC has no zero crossing and can sustain an arc indefinitely. You might not think 12V is much to worry about arcing, but once the arc is established, and the air ionized, it will conduct relatively low voltage over a considerable distance. If the electrical contacts aren’t far enough apart, or there aren’t other provisions to extinguish the arc, the circuit breaker can arc until it melts or starts a fire.

I ended up using standard 100A cartridge fuses. They are only about $7 each which is nice. The connection between the two series batteries was an easy place to mount them, so that’s what I did. With 3 sets of two batteries in parallel, each fused for 100A, that’s 300A total, which is a reasonable margin above the 250A that the inverter might require.

A fuse is really just a piece of wire that gets hot and melts when there is too much current though it. The alloy and thickness of the wire is chosen carefully to give the desired characteristics. When the circuit opens, it is just like opening a switch, so an arc will be formed. I opened up one of these cartridge fuses, and the fuse wire itself is surrounded by a white power, probably baking soda or similar. As the molten metal of the fuse wire drips away, the powder will fall into the gap and extinguish the arc. It’s simple, but clever. I have no concerns about using fuses in a DC circuit.

A different concern is what will happen if I use my house batteries to “jump” the starter battery. Depending on the total resistance of the circuit the surge current could easily blow my fuses. I don’t intend to jump my starter batter hardly ever, so it will remain a concern for another day. A dead starter battery and a dead house battery (due to blown fuses) would really suck though.

I might also connect my house batteries to my starter batteries to charge them all in parallel from the alternator while on an extended trip. In that case all the various batteries would be reasonably charged to begin with, so the surge current when connecting them should be more reasonable, and hopefully not nearly enough to blow my fuses.

One other concern is that one of my 3 sets of parallel batteries might try to provide all the current required by my inverter, in which case 250A could easily blow the fuse. In practice the batteries are all brand new and very well matched so they share the current almost equally. Eventually, as the batteries ware out, they will become more imbalanced, and this will be more of an issue. I will have to monitor their health from time to time. If they ever do get too far out of balance, and it can’t be cured by an equalization charge (controlled overcharging) then the batteries will have to be replaced, because replacing one battery out of a set of worn out batteries is just inviting trouble.

To be Continued….
?

01-16-2010, 10:14 PM	#149
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	400 Pounds of Electrons I now have 6 each 6V batteries, for a total of about 400 pounds. These are top of the line AGM (Absorbent Glass Mat) batteries from Lifeline, GPM-4CT, for a total of about $1,500 with shipping and tax. They had to put them on a pallet and ship them freight but I am starting to get used to that where this project is concerned. This means I have now made my last big ticket purchase. With the exception of the catalytic heater, which I had to send back because I ordered the wrong one, I have all the big ticket items in my possession now. My house is full of stuff that doesn’t belong, like a toilet in the kitchen, and a range in the living room. I am really hoping that my monthly cash flow will turn positive now. Unfortunately, as Blizz will attest, you can spend lots of money on the little things as well. I went on line with McMaster Carr and found most of what I needed for battery cables pre-made. http://www.mcmaster.com/#battery-terminals/=590gck It’s not cheap, but it’s probably a lot easier than making your own. They have cables for paralleling up to 3 batteries, which is what I needed, and they also have cables for connecting the batteries in series. It’s handy that these cables seem to come in just the right lengths which is probably because at 66 pounds each, the batteries I used are about the same size as heavy duty truck starter batteries, so they connect together quite similarly. The lugs on these pre-made cables are cast right onto the wire out of lead. Lead is fairly soft but the lugs are so thick that they seem to be plenty strong. The pliability of the lead probably also insures that it conforms to the mating surface with plenty of contact area. I haven’t noticed any heating even with high currents running through them, so they must be working well enough. I did buy some 2/0 welding cable and solder on connectors for the link from the battery bay to the Inverter bay. I found them here: http://www.mcmaster.com/#catalog/115/781/=590ep3 My experience with the solder on connectors is quite good so far. They were easily soldered on with a propane torch. Just strip the wire, push it into the connector, and heat the connector until the internal solder slug melts. Gentle pressure on the wire will force it into the molten solder and flux making a very strong joint. The solder did not wick up the wire much, so it remained flexible right up to the lug. Depending on the orientation of the lug, some excess solder may drip out, so put something down to catch it. Since the solder joint is up inside the lug, there is no way to inspect it visually. I did a bit of a pull and wiggle test and it seemed very solid though. I also found no heating at the lug even with very large currents running though it, so the implication is that it is a good electrical joint. These solder on connectors aren’t cheap, but for just 4 lugs like this project, they are a lot cheaper than buying a large crimping tool for use with the crimp on lugs. If you already have a crimping tool, or can borrow one, the crimped lugs are probably cheaper. The battery tray I am using is just 1/8” steel, about 22” by 22” and 1.5” deep. It fits the 6 batteries perfectly, which is dumb luck because it came with the truck as a slide in tool shelf. I removed the slides as I don’t think they are strong enough to carry the weight. I also put 6 lag bolts up through the top of the battery box into the wooden floor boards of the living quarters. The battery box is already welded to the steel floor joists, but at 400 pounds I wanted some insurance. I thought long and hard about how to mount the batteries. I wanted them to be somewhat shock mounted to reduce the pounding. I was considering rubber vibration dampers, like motor mounts, but in the end I decided to keep it simple. I set the tray full of batteries on top of one inch of EPP (Expanded Polypropylene) foam with more of the same along the back, sides, and front, set within the door frame. I welded some D rings into the side walls of the box for heavy nylon straps across the top and another across the front to make sure they stay in the battery box. I wanted to have some sort of a fuse or circuit breaker for the batteries, but I calculate the maximum required current from the batteries at close to 250A. I didn’t find any circuit breakers or fuses in that range at the local home improvement store. They might be available somewhere, but I wanted this design to be serviceable using readily available parts when possible. High current circuit breakers can be very expensive. I also think it is dangerous to use a circuit breaker that was designed for AC in a DC circuit. An AC circuit has a zero crossing, or a point in time when the voltage is zero, every 8.3 milliseconds, which is an opportunity for any arc caused by the opening of a switch to extinguish. DC has no zero crossing and can sustain an arc indefinitely. You might not think 12V is much to worry about arcing, but once the arc is established, and the air ionized, it will conduct relatively low voltage over a considerable distance. If the electrical contacts aren’t far enough apart, or there aren’t other provisions to extinguish the arc, the circuit breaker can arc until it melts or starts a fire. I ended up using standard 100A cartridge fuses. They are only about $7 each which is nice. The connection between the two series batteries was an easy place to mount them, so that’s what I did. With 3 sets of two batteries in parallel, each fused for 100A, that’s 300A total, which is a reasonable margin above the 250A that the inverter might require. A fuse is really just a piece of wire that gets hot and melts when there is too much current though it. The alloy and thickness of the wire is chosen carefully to give the desired characteristics. When the circuit opens, it is just like opening a switch, so an arc will be formed. I opened up one of these cartridge fuses, and the fuse wire itself is surrounded by a white power, probably baking soda or similar. As the molten metal of the fuse wire drips away, the powder will fall into the gap and extinguish the arc. It’s simple, but clever. I have no concerns about using fuses in a DC circuit. A different concern is what will happen if I use my house batteries to “jump” the starter battery. Depending on the total resistance of the circuit the surge current could easily blow my fuses. I don’t intend to jump my starter batter hardly ever, so it will remain a concern for another day. A dead starter battery and a dead house battery (due to blown fuses) would really suck though. I might also connect my house batteries to my starter batteries to charge them all in parallel from the alternator while on an extended trip. In that case all the various batteries would be reasonably charged to begin with, so the surge current when connecting them should be more reasonable, and hopefully not nearly enough to blow my fuses. One other concern is that one of my 3 sets of parallel batteries might try to provide all the current required by my inverter, in which case 250A could easily blow the fuse. In practice the batteries are all brand new and very well matched so they share the current almost equally. Eventually, as the batteries ware out, they will become more imbalanced, and this will be more of an issue. I will have to monitor their health from time to time. If they ever do get too far out of balance, and it can’t be cured by an equalization charge (controlled overcharging) then the batteries will have to be replaced, because replacing one battery out of a set of worn out batteries is just inviting trouble. To be Continued…. ?