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Old 08-07-2022, 10:16 AM   #1
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Reducing battery charge time

My motorhome has a Progressive Dynamics 9180 converter with the Charge Wizard. The converter was located in a basement compartment at the left rear of the coach. See attachment 1. The batteries are located under the entry steps on the right side of the coach. See attachment 3. The wires to the converter were 6 AWG. From the converter to +wire went to the coach fuse panel and then on to the batteries. The -wire was attached to a support 1.5" square tubing that was in turn welded to the frame.

If the coach batteries were at 80% and the converter output was 14.5 volts the measured voltage at the batteries was 13V and the charge current was ~20 Amps.

I did a project to reduce the voltage drop between the converter and the batteries and hopefully increase the current flow into the batteries and in turn reducing charge time.

I moved the converter to a compartment inside the coach that is right beside the entry steps. I used 12 gauge ROMEX to add a 120VAC outlet to the compartment for converter input power. See attachment 2.

I used 2 AWG wire to connect the converter to the batteries. See attachment 3 again. Note the Orange wire coming in from the left side, connecting to a circuit breaker and then to the battery control center.

After getting everything connected and making sure all connections were tight I noted the the current into the battery was now ~40 Amps. The converter output was 14.3V and the voltage at the batteries was 14.1V.

That is a really good improvement! Next time I go boondocking I will see if my 1 hour/day generator time to charge the batteries has been reduced. My goal...1/2 hour/day.
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Old 08-07-2022, 11:05 AM   #2
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Nice work.

In case it helps in the future, an easy way to determine where the voltage drop is occurring is to simply put one lead of the voltmeter on one end of the wire and the other lead of the voltmeter on the other end of the wire.

While many people think the voltmeter will read 0 volts all the time, it won't if there is a problem such as an under-sized wire or a poor connection.

I've had people locate charging problems on their toad simply by putting one lead of the voltmeter on the negative terminal of the car's battery and the other lead on the motorhome chassis. If the ground is good and appropriately-sized wire the voltage will be well under one volt. Others, not so much.

Because a digital voltmeter draws essentially zero current to provide a reading you can use a small gauge wire to extend one lead so it reaches where you need it to reach.

This technique can help you find the biggest cause first and then work down from that one to reduce remediation time and costs.

FWIW,

Ray
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Old 08-07-2022, 11:43 AM   #3
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How do you know your batteries were 80% discharged when you measured the current and voltages? Anything other than a shunt based battery monitor is suspect.

If the 80% is correct then the batteries are beginning to stop accepting a charge even though the input voltage is 14.5.

Try it again with the batteries down to a true 50%. You should see close to 80 amps.

What is your battery capacity. You need at least 320 amp hours to routinely charge at 80 amps.

David
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Old 08-07-2022, 12:17 PM   #4
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Wheres a good place to buy thick wire?
Would it be a mistake to use the existing wires and double them up to act as a thicker wire but saves a little money?
Theres an episode of Veritasium i need to watch regarding how electricity flows on wires.
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Old 08-07-2022, 12:30 PM   #5
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Depends on what you mean by thick wire.

Hardware stores and farm & ranch stores will often sell wire by the foot, as will welding supply stores.

I wanted 4/0 cable recently. Local welding store would sell black cable, but I wanted both black and red. I would up ordering on-line, from “wireandcableyourway” or some such web site. That worked pretty well.
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Old 08-07-2022, 12:41 PM   #6
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Those appear to be standard Flooded Lead Acid batteries. Please understand that they charge the slowest and that as they get past 80% state of charge they slow down how many amps they can absorb.

What's happening is that as you increase the SOC more and more internal resistance builds up in the battery's cells. This increased resistance prevents the battery from accepting a large amount of amps from your charger.

You can increase all the gauges of all the wiring but you can't stop the internal battery resistance from limiting the charging above 80% SOC.

Assuming those are 6v deep cycles, they are the slowest to charge - because of the thick plates inside - and will be the most resistant to high charge rates as they near 12.7v.

I'm not saying you should do this, but Lithium batteries don't exhibit this increased resistance at all. Hence, they are the fastest charging battery type. 12v AGM's are a little better than 6v FLA's but only a little.
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Old 08-07-2022, 01:57 PM   #7
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Something in support of the OP's notion to reduce resistance between the converter and the batteries isn't so much what happens during absorb phase, because that's when current is less and it's a fixed duration anyway. The benefit is to reduce bulk time and it will also more accurately reflect terminal voltage back to the converter to accurately sense the absorb voltage setpoint.

What can potentially happen when there is some impedance between the converter and batteries is the voltage at the converter will be higher than at the batteries. Absorb phase in a multistage converter is timed, based on when the converter senses the voltage reached ~14.4V (bulk complete). With a resistive feed the voltage sensed at the converter will reach the absorb setpoint before that voltage is reached at the battery. So the absorb phase timer starts too soon, before the battery has finished bulk phase and then may not run long enough for the batteries to actually finish absorb phase. The converter will switch to float and the batteries will continue to charge, albeit at the trickle rate. The end result is unless you let the batteries float long enough to restore the charge not supplied by the short-cycled absorb phase, the batteries will not be at 100% even though the converter completed the charge cycle.

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Old 08-07-2022, 02:24 PM   #8
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Thanks for all the replies. Most of the replies are aimed at FULLY recharging the batteries. That is not my goal. I go on 4 or 5 day boondocking trips. I want to recharge the batteries to the extent that I can do my usual activities using battery power while having less generator time. What my changes do to fully recharge the batteries while plugged into shore power is not a consideration.

I managed to lower the voltage drop from the converter to the batteries from 1.5V to .2V and increase the charge current from ~20 Amps to ~40 Amps. I haven't field tested it yet but I think I can now get the energy I need from the batteries under those conditions and use the generator less...possibly half as much.

My before and after tests were performed under as similar conditions as I could get. The batteries were fully charged with the float voltage setting at 13.3V and the float current at ~.1 Amp. I discharged the batteries until I had used about 500 Watt Hours. 500 Watt Hours is ~ 40 Amp hours or about 20% of my 230 Amp Hour rated batteries.

I know 1/2 hour at 40 amps is not enough to fully recharge the batteries. That isn't my goal. The goal is to replace enough energy into the batteries so after 4-5 days I still have more than 50% charge in the batteries. I know I will start each successive day with less charge than the previous day.

From experience I know that 1 hour generator per day charging at 20 Amps for 1 hour was enough to achieve my goal. I think 1/2 hour per day at 40 amps may do the same thing. We will see when I get the first chance to field test it.

I already had most of material necessary to do this change. I spent ~$60. If I chose to go lithium it would have been $hundreds. Maybe if I stayed out a month at a time that might make sense. For 5 days I like $60 much better.

I am just trying to optimize what I already have.
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Old 08-07-2022, 02:32 PM   #9
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Quote:
Originally Posted by wopachop View Post
Wheres a good place to buy thick wire?
Would it be a mistake to use the existing wires and double them up to act as a thicker wire but saves a little money?
Theres an episode of Veritasium i need to watch regarding how electricity flows on wires.
It would work but in my case the coach fuse panel is back near the left rear where the converter used to be. I still needed the existing wire from the batteries to the fuse panel. Besides, getting the wire out in my case would have been VERY difficult! I just left it in place. In fact, the only difference is the shorter, bigger wires to the Battery Control Center and the 80 Amp circuit breaker. Every thing else is still as designed by the coach builder.
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Old 08-07-2022, 05:27 PM   #10
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Thanks for the tip on this. I am considering adding a inverter/charger to invert my outlets when boondocking. I think the placement will give me this added benefit of a faster charge. This confirms what I thought. Thanks
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Old 08-07-2022, 07:52 PM   #11
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Don't lose any sleep if you drop below 50% SOC.

Lead acid batteries can easily handle deeper discharges, down to 20% SOC.

The cycle life is close to linier between 80% SOC down to 20% SOC. The myth that your harming your batteries by going below 50% SOC is just that, a myth.
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Old 08-08-2022, 06:55 AM   #12
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Quote:
Originally Posted by twinboat View Post
Don't lose any sleep if you drop below 50% SOC.
This coming from a guy that makes inverters cry...

Mark B.
Albuquerque, NM
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Old 08-08-2022, 11:54 AM   #13
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Quote:
Originally Posted by Ohiomike View Post
Thanks for the tip on this. I am considering adding a inverter/charger to invert my outlets when boondocking. I think the placement will give me this added benefit of a faster charge. This confirms what I thought. Thanks
Hi Mike,

Inverter input and charger output both involve large current flow. Having your inverter/charger physically near the batteries and connected with big wires will definitely help it perform to its maximum potential.

I only use my inverter to run my entertainment center. Maybe 6-10 Amps maximum. I used relatively small wire and the inverter is maybe 15' away. I lose about 1 volt in voltage drop from the battery to the inverter. That is OK for my inverter. It works fine for its intended purpose.
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Old 08-08-2022, 12:18 PM   #14
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Quote:
Originally Posted by twinboat View Post
Don't lose any sleep if you drop below 50% SOC.

Lead acid batteries can easily handle deeper discharges, down to 20% SOC.

The cycle life is close to linier between 80% SOC down to 20% SOC. The myth that your harming your batteries by going below 50% SOC is just that, a myth.

Agree completely!


That being said....Your inverter might not be so happy as your battery voltage drops (especially if you are pulling heavier 120v loads through your inverter).
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