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Old 06-11-2020, 05:41 PM   #1
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Battery plan for new guy /

Hi
  • I bought a 2019 (13 months old) SLX 175 RD Baja. Great shape (GENERALLY) because it almost was not used.
  • I tow with a little 2015 Tacoma w/ Tow (hd battery and alternator).
  • I will occasionally boondock for a day or two, WITH a 3400 W inverter/gen.

Unfortunately, I don't have direct access to the trailer right now and have needed to do short 15 minute bursts of investigation until this weekend when I will spend a couple days with it. That generates some of these seemingly simple questions.

Today's question:
I was able to look today at the 12v battery. It is the typical Interstate SRM 24 Deep Cycle, 13 months old. It probably has not been well taken care of in the last 9 months because the trailer hasn't been used. I did NOT have my multimeter with me, but the dummy tester on the console showed "1/3rd". So ...

1) I may need to replace it, but will test again more accurately to see how far it is really drained. If it is reading lower than 12.2 should I plan on replacing or give charging once a try?
2) How long does it take to charge from 50% to "Full" using Tow V, Shore Power, Inv Genny?
3) If I do need to replace it, does my usage justify a 27? Does it really matter? I don't think I need to go to dual batteries.
4) I don't see myself using a trickle charger or similar while stored ... but I would not be against disconnecting or adding a quick disconnect switch. Is that futile effort?

In general, I don't want an expense I don't need, and I don't want weight I don't need. Thoughts and advice?
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Old 06-12-2020, 12:09 AM   #2
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I'll try to help out on some of your questions here.
1) 12.2 volts is about a 50% state of charge (specific gravity of 1.190 by a hydrometer) A deep cycle battery can be "deep cycled" a limited number of times, no more than 30, and since it's relatively new, I would try to charge it, but not "burn" a charge into it with a huge charger.
2) A 10 amp charger will bring it (at 50% charge) to full charge in about 10 hours or so. "Full charge" being 14+ volts, and after it rests for a few hours, will stabilize at 13.6 volts (1.265 SG by hydrometer). The wires from your truck are very small, and will take a very, very long time to charge it. An automotive alternator (unlike the olden days generators) is a battery "maintainer", not a charger. I can't comment on the genny, or shore power/converter charger for a charging time. I do know your converter will have at least a three stage charger, like a Battery Tender. It won't overcharge a battery on you.
3) I would give it a chance before replacing it. It probably will come back. If you do need to replace it, a gr 27 is fine, you'll just need to spend another $15 on a bigger battery box. We don't boondock, and our gr 24 Interstate is the original battery (knock on wood) and is 8 years old.
4) My first choice on maintaining a fully charged battery when it's not being used, is 1) plug the TT into shore power. 2) Hook it to a Battery Tender, or similar. 3) Disconnect both cables on a fully charged one. A quick disconnect can still allow a parasitic draw from the propane/CO detector and radio. 4) Remove it, take it home, and THEN hook it up to a Battery Tender. (Can you tell I like my Tender??)

Batteries need some attention, and hate to be neglected. Good luck on bringing yours back to life. Keep us posted on its prognosis!
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Old 06-12-2020, 07:28 AM   #3
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If you get a chance, unhook the battery till you can get it charged and Get a disconnect switch, your trailer has parasitic items that slowly drain your battery. I have a voltage monitor in my trailer, when unplugged from shore power it always shows between 12- 12.5 volts, on shore power it reads 13.4 , my 2 batteries runs everything for days.. sounds like you may just need to service( check water levels), then charge your battery back up and go enjoy it. If it does fail, batteries are plentiful and can be found at almost every town you come across.
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Old 06-12-2020, 08:03 AM   #4
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Thanks guys.

Question about the below:

Quote:
Originally Posted by JFlightRisk View Post

3) Disconnect both cables on a fully charged one. A quick disconnect can still allow a parasitic draw from the propane/CO detector and radio.
I understand parasitic draw of course ... but how on earth can the CO detector or radio be drawing power from a battery with the negative cable disconnected?
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Old 06-12-2020, 09:32 AM   #5
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It can't as far as I know. I am X2 on the Battery Tender Jr. will keep a battery working like new all winter.
I don't think anyone mentioned but batteries may lose water when charging. Yours may already be down. Gently pry up the two oblong covers and take a look. Water should be about 1/4 inch above the top of the plates on all cells. If one cell is really low and the rest are good it may indicate failure or soon to be dead. USE ONLY DISTILLED WATER! Minerals in tap water or bottled drinking water are bad for the battery.
Lots of posts on battery maintenance, winter storage, etc. available on the search bar above.
A Group 27 gives you a little more leeway as to boon-docking use.
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Old 06-12-2020, 09:43 AM   #6
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I've seen some disconnect switches are installed inline on the positive cable, and some with the negative. There's an easy test to see if there is indeed any draw, once the negative cable is disconnected. One valuable tool in your RV toolbox is a digital Volt-Ohm Meter (VOM).

Take the negative cable off the battery. Now you will have an open DC circuit. Set the VOM to the 10 amp setting, and touch the positive lead of the VOM to the cable end, and the negative lead to the negative battery terminal. The meter will now be completing(closing) the circuit. This will show you if there's any parasitic draw from the battery.

If there's any amp, or (most likely) milliamp reading on the VOM, there's something drawing power. Now to find out where it's coming from, at the fuse panel in the TT, start pulling 12 volt fuses out one at a time, watching the meter for any change. Now for the kicker...there's the possibility there's more than one circuit with a draw, so if you pull a fuse and the meter changes, leave that one out and continue on until the VOM reads "0".

The most common draws are from the radio, CO detector, antenna booster that's been left on, the microwave clock, or a compartment light left on.

I do hope some of this helps out. Electrical diagnosis can be a pain, second only to a water leak!

If you have any questions, please ask, and I'll try to help.
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Old 06-30-2020, 02:35 PM   #7
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minor update:

Last week we were able to take a long trip, mixture of shore power and generator backed boon docking. Through empirical evidence as well as my multimeter testing ... it looks like this battery is toast and there is no major parasitic draw. (I did learn what the antenna booster was though!)

I will replace the battery when we head out again in a couple weeks. Going to be honest, I think a type 24 SRM will be fine. I do plan on attaching a simple quick disconnect. I am (perhaps unfortunately) unlikely to lug the battery or keep it on a tender while in storage, so a quick disconnect is about the best I can expect myself to manage.

Question: Are these cheapies ok for this purpose:

https://www.harborfreight.com/batter...tch-97853.html

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Old 06-30-2020, 04:26 PM   #8
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thats the one I use. dead simple, pretty much failure proof. I would still consider upgrading to a group 27 for a little more capacity. Just shopped for one and sams club was the best price I could find.
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Old 06-30-2020, 04:34 PM   #9
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Thanks Roger. I will need to look at the battery box when I am near the trailer to see if a 27 will fit (doubt it). If it does I may go that direction. Otherwise, not sure I am willing to do hassle with a box swap.
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