How long to run deep cycle batteries?

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versicolor

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Apr 6, 2015
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Hi all,
I am planning a trip this year, and would like to do lots of dry camping.
I'm rethinking the power situation in the camper. Currently, it's got an old deep cycle battery.

I would like to replace the battery with two 6v T-105's.
The question is, I have always just camped until the lights & fan start to get dim/slow.
Obviously, I need to take proper care of the nice new batteries, and I've heard I shouldn't run them below 50% depth of discharge.
So, how do I tell the DOD %?
Out of curiosity, does the lights running dim mean it reached 0%?
I can install a voltmeter, but it seems to be super inaccurate due to activity.
Thanks
 
Here's a table with reletive numbers to get you started. If you're waiting until the lights get dim to recharge you're waiting way to long IMHO. These numbers are for batteries at rest. Obviously if you have or have had a heavy load on you'll get an erroneous reading. But, somtime during the day I'm sure you'll find a time when there is no or little load. I would suggest a digital voltage guage for ease and accuracy of readings.

State of Charge Sealed or Flooded Lead Acid Gel battery AGM battery
100%                                 12.70+                   12.85+           12.80+
75%                                         12.40                   12.65           12.60
50%                                         12.20                   12.35           12.30
25%                                         12.00                   12.00           12.00
0%                                         11.80                   11.80            11.80

Another option which is more expensive by far, is a shunt based system to monitor how much energy you've taken out of the batteries and then how much you've put back in.

Ken
 
I think everything Ken said is correct but I'll expand a little on battery monitors. We do a fair amount of boondocking so I installed a Trimetric RV 2030 battery monitor. It's a device like Ken mentioned that's installed with a shunt. It is, hands down, one of the most useful things I've installed in our RV.

It not only displays voltage levels for two battery-banks, it also displays the amount of amps that are going into or out of a battery-bank. That's useful, because you can see exactly how much power a particular device draws, or how well your batteries are charging. It also displays (in percentage), how much battery power you've got left in your batteries. I find that feature to be very useful when dry-camping. At about $130.00, it's definitely more expensive than a basic volt-meter, but it's also much more informative and useful.

Kev
 
Great, thanks.
So to be clear, I should probably recharge once it drops below around 12.2 rest volts.

On an unrelated note, do you think the installed 'power center' should be replaced? It's an old PD 723q from early 80's.
I feel like a proper 3-stage charger would be better for the life of the battery. Also, it gets really hot. Do you notice your converters getting hot?
cheers
 
Batteries can be permanently damaged if they're fully and repeatedly discharged, and some types of batteries are more susceptible to damage than others. It's been my practice to not allow my batteries to get below 60%, and even that's infrequent. Good batteries are expensive so taking care of them is important.

Along those lines, it's my opinion that having a good (smart) charger is crucial to taking care of your batteries. There are still a lot of two-stage chargers out there and, while they do work, it's so easy for them to over charge your batteries and irreparably damage them. You really have to monitor them closely to keep that from happening. When I plug into shore-power after a deep discharge, my charger shows a charge-rate of 120 amps and a pretty big cooling fan kicks on (It's thermostatically controlled). Yeah, converters can get pretty warm when they're in heavy charge mode.

Kev
 
Yes on both counts. 12.2 is a good time to recharge and a 1980's vintage charger is not a good choice to continue with. A modern 3 or 4 stage charger will give better battery performance, IMHO.

Ken
 
I've heard I shouldn't run them below 50% depth of discharge
Click to expand...

I have a little different perspective, based on 10 wet cell golf cart batteries in our boat and 4 in our coach. Golf cart batteries are made for repeated deep discharge, and occasional discharge below 50% charge is not going to kill the batteries.
 
Yeah the way I see it, the occasional 30% won't be too bad if it's charged back up relatively soon after.
The deeper the cycles, the lower the cycle count, I guess. But nothing to stress about.

As for the power center, do you think there's any way to add the 'smart' charge capability to it? Or should I just replace the whole thing? It would just be a pain since the distributor panel is all wired in and such.
 
The only thing a smart charger will get you is higher bulk current which means faster charging.

The Magnetec 7300 series are fully regulated and put out a constant 13.5 volts output.  This is what you want to avoid overcharging the batteries and boiling them dry.  The disadvantage is the charging current will taper off sooner than a smart charger that boosts the voltage to 14 or 14.5 volts while the batteries are charging, then drops back to 13.5 volts after the batteries are charged.

The older Magnetec 6300 series converters are the junk you want to get rid of.  They send unfiltered power to your lights and appliances, take forever to charge your batteries and then once they're fully charged it will boil them dry.
 
interesting. you don't happen to know about the progressive dynamics pd723 do you? i can't seem to find any info on it.
i just replaced the lights with led, so unfiltered power would be not good.
 
The charge table that bucks2 posted is quite conservative. Attached below is another widely used one that shows 12.06v 12.6v for the 50% level in a lead acid battery such as a golf cart deep cycle. And the GC2 batteries are quite tolerant of deep discharging and I wouldn't be at all afraid of pushing that 12.06 down a bit lower.

Be aware that even a slight background current draw while measuring can decrease the observed voltage reading by 0.1-0.2v, so make some allowance for that.

We don't boondock often, but when we do my 4x Trojan AGM 12v deep cycles get down to around 11.6v by early morning (with the inverter on overnight). They are now almost 8 years old, so don't seem to have suffered much.

There is a huge difference between what can be measured in a lab environment and what makes a practical difference in your RV. There are so many variables in actual use that it pointless to obsess over just one aspect. By all means be kind to your batteries, but don''t make it a life-or-death concern.
 

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versicolor said:
interesting. you don't happen to know about the progressive dynamics pd723 do you? i can't seem to find any info on it.
i just replaced the lights with led, so unfiltered power would be not good.
Click to expand...

My apologies - I misread the model number you posted earlier!

The PD 723q is one of the bad chargers.  When it was made, it wasn't possible to produce anything close to the fully regulated multi-stage converter/chargers we have today.  It's functionally identical to the Magnetek 6300 series that everyone hates, sending unfiltered power to your house circuits and a very primitive low current battery charger that takes forever to charge the batteries then continues to boil them dry if it's left connected to shore power.

Here's a link to the service manual:  http://www.progressivedyn.com/pdfs/PDI%20Service%20Manual%20710778.pdf

Physically, it looks like the 723q would be difficult to modify for a replacement converter.  Another issue is the tubular glass 12 volt fuses it uses are getting harder to find, everyone's switched over to the dual blade style fuses.

I think you'd be better off replacing the whole power panel instead of trying to modify what you have.  Something like this would work:

Progressive Dynamics 4000 series power panel

It includes a 45 or 60 amp IntelliPower converter/charger with Charge Wizard to provide fully filtered power to your lights and appliances and treat your batteries right.
 
Ok, so I replaced the power center with a PD4045, and the battery with two 6volts in series. Everything works.
Although, before the overhaul, the led bulbs used 0.31amps, now they draw 0.41. All the appliances have a similar increase in amp usage.
Any ideas as to what could cause this?
 
You're running slightly more voltage with the new batteries and power center.  More voltage = more current.  Congratulations!
 
Tinmania said:
I would think more voltage = less current.

Why would it be more?
Click to expand...

Ohm's law.  E(voltage) = I(current) * R(resistance) or I = E / R .  R is constant (for our purposes anyway) so an increase in voltage (E) means a corresponding increase in current (I).  This only applies to purely resistive loads, like light bulbs.
 
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