How to make a cheap isolated dual-battery setup for $50

[dwh]

When you combine your start and house batteries, due to your start being fully discharged (otherwise known as self-jumping), isn't the current flowing between them going to be crazy high? Is that bad?

No. The resistance curve of a lead-acid battery is low in the center, high on both ends. Meaning that the resistance is higher as the battery approaches full, and also higher as it approaches dead.

So connecting a full battery to a dead battery doesn't result in a big surge or a heavy amp flow.


However, after you do that and then engage the starter, the starter will draw what it needs from the aux battery.

 

[carbon60]

No. The resistance curve of a lead-acid battery is low in the center, high on both ends. Meaning that the resistance is higher as the battery approaches full, and also higher as it approaches dead.

So connecting a full battery to a dead battery doesn't result in a big surge or a heavy amp flow.

However, after you do that and then engage the starter, the starter will draw what it needs from the aux battery.

Thanks very much for that explanation, clears things up for me.

A.

 

[ducktapeguy]

Yes, I've done it for years. I have a deep cycle battery in a box that I move from vehicle to vehicle, where ever the fridge is going. I've wired a 7-pin trailer plug to the charge circuit only (2 wires). The plug goes into the trailer socket next to the hitch, and through the back hatch. When the engine is running, the battery charges. When off, it doesn't. I've also installed a volt meter into the box to keep track of the battery charge, a bunch of accessory ports, and a 800w 110v inverter on the lid. It's a bit ghetto what with the wire pinched in the door, but it works just fine. If I really had my act together, I'd wire a permanent socket off of the same trailer battery charge circuit into an access panel inside the vehicle.

Some will say that the 8-10 gauge wire and 40 amp fuse won't allow full charging or fast charging, and they may be right. But like I said, I've used it on and off for years and it works.

When you combine your start and house batteries, due to your start being fully discharged (otherwise known as self-jumping), isn't the current flowing between them going to be crazy high? Is that bad?

I have a similar setup to power my fridge. 10ga wires running to the rear cargo compartment, connected to a battery box which in turn powers the fridge. I initially had the same concern about the current spike on startup and thought I would need to limit the current, but after some testing I've found there's not much to worry about. Even when the fridge shuts down from the low voltage cutoff, the resting voltage of the aux battery is still about 11.2V. Normally I drive everyday so it shouldn't get down that far, but this last weekend I was on a road trip and didn't drive much for a few days, so it did hit the low voltage cutoff. At engine startup, the initial current to the depleted aux battery is less than 12A, and will drop fairly quickly to <9A. Worst case, if I start the engine while the fridge is running and the battery is right above the cutoff voltage, in theory it should draw about 18A max, but in reality I haven't seen more than a 13A draw.

I have the rear circuit running on a relay triggered by the accessory circuit, so power doesn't start flowing until after the engine is started, so that hopefully prevents the current backfeeding into the starter. As a preventative measure, there are 3 fuses inline (30A under the hood, another 30A at the aux battery, and an 80A circuit breaker powering the fuse box) to prevent problems in case either battery get discharged below the minimum voltage.

 

[kjp1969]

I got the idea for my setup from our camping trailer. We had two 6v golf cart batteries in series to power the trailer, charged either by shore power or the trailer battery charge circuit. It worked fine there, so I just adapted the idea to a portable battery box. Maybe it doesn't charge as well as a properly engineered dual battery system (I'm no diesel mechanic) but I haven't seen any downside, and the portability between two cars is a bonus. It seems like the hard work of engineering a engine-on-only charge circuit has already been done, so why not take advantage of it.

 

[peculierboy]

How do I size the wire and fuses for my RAM 4500 with dual 380 amp alternators? Each one is 190 amp. I want to charge the house batteries (2 6v 220Ah each) in my truck camper.

Guestimating I'll have about a 50' run (round trip).

I have no idea how much 'need' the house batteries will require. I'm assuming I'll require #1AWG or #1/0AWG wire for that length, so do I just use 175 amp fuse for #1 and a 200 fuse for #1/0? Or do I need bigger wire due to the alternator output? Or will #2 AWG work fine instead?

I'd like to use Anderson connectors....SB175 or SB350?

 

[john61ct]

Can't be that casual with such high current, 2/0 is unwieldy, and good quality wire that fat is expensive

Get your bank 40% depleted and see what current it demands using a short run of 4/0 from the alt running at say 3000rpm

Actually measure the run with string, as short as possible but ideally a low temp path

 

[john61ct]

Once you know the AWG, its maximum rating dictates the fusing, rounding down.

 

[DiploStrat]

If you derate the alternators to 200A (because they will never give you 380A, at least not for long enough to worry about), drop the voltage to 14v, and assume 25 feet one way, then a 1/0 AWG cable will give you:

7% drop or 13v at the battery. (https://www.calculator.net/voltage-...ce=25&distanceunit=feet&amperes=200&x=79&y=26)

Sounds terrible, but assuming that your discharged battery is at about 12.5v, it ain't bad. It will start charging nicely.

Now as the battery voltage rises and the amp flow drops to about 100A, you get 13.5v at the battery. (https://www.calculator.net/voltage-...ce=25&distanceunit=feet&amperes=100&x=68&y=20)

Finally, when your amp rate drops to 50A, you get 13.7v at the battery. (https://www.calculator.net/voltage-...ce=25&distanceunit=feet&amperes=100&x=68&y=20)

And when you get the the absorb stage and your amps are down to 25A, then you get 13.8v at the battery. (https://www.calculator.net/voltage-...ce=25&distanceunit=feet&amperes=100&x=68&y=20)

Thus, a 1/0 AWG cable is probably the minimum you should use. (I used 2x1/0 AWG, or about 100mm2 of copper. Fuse at BOTH ends.

And add at least 200w of solar with a good multi-stage controller. I had very good results with a Blue Sky MPPT model, but some swear by Morningstar. Solar or shore power is almost always required to complete the absorb stage and assure that the battery is really fully charged.

 

[peculierboy]

Thus, a 1/0 AWG cable is probably the minimum you should use. (I used 2x1/0 AWG, or about 100mm2 of copper. Fuse at BOTH ends.

And add at least 200w of solar with a good multi-stage controller. I had very good results with a Blue Sky MPPT model, but some swear by Morningstar. Solar or shore power is almost always required to complete the absorb stage and assure that the battery is really fully charged.

Thanks, this helps alot. I have a solar system as well.

 

[peculierboy]

Can't be that casual with such high current, 2/0 is unwieldy, and good quality wire that fat is expensive

Get your bank 40% depleted and see what current it demands using a short run of 4/0 from the alt running at say 3000rpm

Actually measure the run with string, as short as possible but ideally a low temp path

Thank you

 

[luthj]

Good advice above. It depends a lot on the wiring and the specific alternator. However, many small case bosch alternators can and will provide their rated amperage at speeds above idle. Especially in moderate ambient temps. The bosch internally regulated alternators with internal fans (from the late 90s to present day) use a digital internal regulator which monitors the alternator temperature, and will derate and/or disable the alternators output if a overheat condition is present. This does not completely eliminate overheating issues, but does mean there is less concern.

So, assuming good airflow/cooling you can expect up to 300A or so from the alternators is the loads/batteries will accept it.

Your 220AH bank will probably never accept more than 100-150A at alternator voltages. But if you have a higher voltage alternators (Dc-Dc charger/external regulator) you may see 200A for brief periods.

The key to quick alternator charging is to keep the voltage drops as low as possible. Big fat cables for a low resistance path is ideal. Many vehicles alternator harnesses have too much voltage drop, so I prefer to connect directly to the alternators output when possible. Many alternators have an unused output stud (it may be short). The same goes for the engine to chassis ground strap. Many are high resistance, so a quality ground cable between the engine block and chassis is often called for. Given your length of run, a chassis ground will save you some money on heavy cable.

Note that when connecting directly to the alternator, you will need to fuse the cable near the alternator unless the alternator to chassis battery cable has a fuse (some do). A fuse will also need to be placed near the house battery bank.

If you can keep the voltage drop under 0.5V at 75-100A you will have an excellent charging experience. Take note that most alternators do not have high enough voltages to complete the charging process, (14.4+) so a finish charge via solar, shore power etc is generally required to finish the absorb period.

 

[fsquared]

Great write up order most of the parts you recommend. Shame the solar panel was of the page. I was looking to finish the suburban off with those.

 

[fsquared]

Possible group by on those controllers https://www.ebay.com/itm/MorningSta...0.XSunSaver+10+Charge+Controller+12V+10A.TRS5

 

[DiploStrat]

Good advice above. It depends a lot on the wiring and the specific alternator. However, many small case bosch alternators can and will provide their rated amperage at speeds above idle. Especially in moderate ambient temps. The bosch internally regulated alternators with internal fans (from the late 90s to present day) use a digital internal regulator which monitors the alternator temperature, and will derate and/or disable the alternators output if a overheat condition is present. This does not completely eliminate overheating issues, but does mean there is less concern.

So, assuming good airflow/cooling you can expect up to 300A or so from the alternators is the loads/batteries will accept it.

This matches my experience with the two 125A small Bosch alternators on my Chevrolet. There has been, so much viewing-with-alarm about alternator output on this forum, that I decided to "derate" my expectations. ;-)

Your 220AH bank will probably never accept more than 100-150A at alternator voltages. But if you have a higher voltage alternators (Dc-Dc charger/external regulator) you may see 200A for brief periods.

This is absolutely true, as a practical matter, the voltage of a decent battery will rise so rapidly that most of the time the actual charge rate will not exceed 50A or so. (Which, of course, is why so many B2B/DC-DC chargers can work with such low output. REDARC, for one, appears to compensate for a maximum of 40A output by boosting their lowest voltage to 14.6v.

The key to quick alternator charging is to keep the voltage drops as low as possible. Big fat cables for a low resistance path is ideal. ... Given your length of run, a chassis ground will save you some money on heavy cable.

Absolutely. It is often much more practical to work with two or more smaller cable than to try to work with a 4/0 AWG or similar. Copper is copper so the results are the same. A chassis ground will work, as long as it is, in fact, a good ground.

If you can keep the voltage drop under 0.5V at 75-100A you will have an excellent charging experience. Take note that most alternators do not have high enough voltages to complete the charging process, (14.4+) so a finish charge via solar, shore power etc is generally required to finish the absorb period.

I have gotten a bit more modest in my wiring specs as the charge rate drops so rapidly and the really high voltage is only needed below 50A or so. (This is the "secret" of the CETEL D250S/SmartPass combo - relay until the charge rate drops to 20A and B2B there after.

Ignoring "smart" alternators, which which I have no experience, the world seems to divide into two camps, the 13.9v and the 15v and over. Sprinters and some Toyotas are in the former and Chevrolets and some other "Big Three" trucks appear to be in the latter. You really cannot discuss this issue until you know the voltage range of your vehicle.

 

[kjp1969]

My experience is limited to my travel trailer that also had dual 6v in series, and our Toyota Sequoia charged them just fine (or so I assume) from the 40amp charge circuit that was built in to the truck. Maybe it would charge slower, I don't know. Serious question, nothing smart alec about it, but what's wrong with just using the factory charge circuit? Why is running an entirely new circuit with massive wires necessary or desirable?