Charging from truck

[Eldough]

Hello All! I wanted to see if I could get some feedback from those here who have had experience with charging their camper battery from the truck battery. I have read through many previous posts and have learned a lot…but I couldn’t find anything addressing my situation directly. Quick background: we previously had our camper on a 1995 tacoma and charged the camper battery from the truck battery running 10awg wire from the truck battery about 15' to the camper battery with a surepower 1314 to separate them...this setup would charge the battery to a full resting charge of 12.8V. Then we switched to a 2009 tacoma and now my camper battery is only getting charged to about 12.5V.

Currently, with the camper battery at 12.5V and not connected to the charging system, I'm seeing only a 0.1V drop at the point just before the camper battery...but when the camper battery is connected, I believe the alternator isn't seeing the 12.5 volts at the camper battery...only the 13.9-14.1 at the truck battery...therefore I think the alternator is not putting out the amps I need to charge my camper battery?? We have a 130A alternator, so I don't think we need to go the dual alternator route and we also have a solar panel, so I don't need to really pull that many amps...just want to keep the battery topped while in transit. I’m thinking the DC-DC charger that Pods8 has installed may fit the bill, but wanted to see if anybody else has experienced this? Other forums on the web have discussed this and the consensus seems to be larger gauge wire...up to 4AWG??...so that the alternator can sense the voltage at the camper battery. Any input would be greatly appreciated. Thanks!

 

[HERR42]

when the isolator connects the load to the truck charging system the alternator's regulator will attempt to maintain voltage levels by providing current. you already know this. if the load is great enough both the charging system and the truck battery will attempt to provide current as long as there is a voltage difference. once the voltage difference is reduced the amount of current flow drops and the load voltage will rise. at some point parity is reached and current flow hits a minimum.

i have never seen a truck charging system that only went as high as 12 volts.

as an example my truck when started will run for about 30 seconds before activating the charging system and will supply 14.5 volts.

you should see something similar.

you should see this charging voltage at the camper battery ... maybe not while the camper battery is presenting a load and being charged but once its voltage is brought up above 12.8 it wont accept as much current and you should see the truck charge voltage.

the specs on the surepower unit say that it wont connect the load until the source voltage reaches 13.2 volts.

have you measured the voltage at the camper battery? while the truck enginge is running? and given enough time for the charge system to go active? i cant believe the truck only generates 12.5 volts.

i hope your using a reliable meter.

and 10g wire is sufficient.

 

[pods8]

As mentioned you alt is never going to directly sense what voltage is getting to your camper battery, it seeks to maintain the vehicle battery. However when you draw off the vehicle battery to charge your camper battery it should pull amps and draw down the vehicle battery voltage causing the alt. to increase output as needed.

It'd check the voltage along your wiring to see where you get the drop off (ie does it happen mid run, across the surepower, or just at the battery?) because that just seems way to low. A loose/corroded connection or such increasing resistance could be your culprit.

I like the DC-DC charger because it will make up for some voltage drop as well as charging the battery at the correct bulk and float rates for my AGM batteries and ideally longer life than just giving them whatever the alt throws out (looking through some notes it is supposed to bulk charge at 13.7V, over charge at 14.4V and float at 13.2V). However I still think your getting too much drop in general for some reason that really should be addressed/investigated.

 

[Eldough]

Thanks for the input Jeff and Pods8!...I really appreciate it. So I did a test...I started this morning from scratch after not having run the truck for three days (I also by-passed the surepower separator because as you mentioned, it has a cut-in voltage of 13.2 volts and a 10-second delay, so it would constantly connect and disconnect once the load (second battery) was present). Measured 12.66 at the truck battery and 12.55 at the camper battery...not connected. I also measured 12.64 just before the camper battery not connected (=0.02V drop in wiring). Keeping the batteries separated, I then started the truck and immediately read 14.21 at the truck battery. I then connected the camper battery and it slowly started to rise to 12.60 within 1 minute. After 5 minutes of running, I read 14.01 at the truck battery and 12.67 at the camper battery. After 10 minutes of running, I read 13.88 at the truck battery and 12.70 at the camper battery. After 25 minutes of running and putting a small load on the camper battery to draw down a bit, I read 13.87 at the truck battery and 12.56 at the camper battery...and it appeared to slowly continue to drop with the small load attached. So I would guess there be a certain voltage at the camper battery that would cause the voltage at the truck battery to be low enough for the alternator to send more amps? Thanks again.

 

[pods8]

So I would guess there be a certain voltage at the camper battery that would cause the voltage at the truck battery to be low enough for the alternator to send more amps? Thanks again.


The camper battery being lower will draw down the truck battery and cause more amps. The greater the difference the higher the driving force for the amps, and the lower the difference the amps will taper off. Probably not the exact answer you were looking for but its the reality.

However the rest of your post got me curious, how old is your battery and what kind of battery is it? With it directly connected it seemed like things started behaving properly till that small load seemed to over power things (I'm assuming only the camper battery continued to drop and not the truck battery here). I'm curious if your battery in on its final legs and really doesn't have much charge capacity left. Do you have a normal shore power charge (110V)? You could charge up the battery with it then after let it stabilize for a while and verify its still holding a full voltage, then put a load on it for a bit to see if that load draws it down really fast or if it tends to draw down like normal.

 

[HERR42]

i think the battery is hosed...it should come up above 13 volts in that amount of time.

 

[Eldough]

The camper battery is an Optima blue top about 2.5 years old. Most of the time it sits on a batteryminder 12118 (been off since I've been trying to figure out this charging). When charged on the batteryminder or solar panel, it will have an overnight resting voltage of about 12.7 and stay there for a long time. When fully charged, it powers our engel fridge (pre-cooled on 120V) for about 48 hours down to 12.2 volts resting. I think maybe the next step would be to measure the amps and see how fast they taper off, but I don't have a DC amp meter. I'm taking the camper battery down to 12.3 right now and will run the test again. Again, appreciate all the input! Cheers!

 

[HERR42]

i have one too! a D31M

full charge spec is 13.1v

hmmm...same age.

 

[Eldough]

i have one too! a D31M

full charge spec is 13.1v

hmmm...same age.


Your's 13.1 after 24 hours? If so, I think you guys may have found the weak link!

 

[Eldough]

So I ran another test. Got the camper battery down to 12.44V and the truck battery rested down to 12.83V. With the batteries disconnected, started the truck and measured 14.18V at the truck battery immediately after starting. Let the truck run until the truck battery was down to 13.84V...about 15 minutes. Connected the batteries, again without the surepower. Measured 13.81V at the truck battery and 12.48V at the camper battery...which was slowly starting to rise. So I decided to put more of a load on the truck battery to see if I could get the alternator to put out more amps...I turned on the headlights, hazards, blasted the radio and AC. After 10 minutes, the truck battery was down to 13.44V and the camper battery was no longer creeping up. I disconnected the camper battery and measure the volts just before the camper battery = 13.11V...looks like a 0.3V drop when all the stuff is on. From this information, it sounds like my alternator is only putting out enough amps to keep the truck battery above a certain voltage. I was hoping to see 14.1V at the truck battery when all the "stuff" was on, but it didn't get there...basically stayed around 13.44V while I was observing...at some truck battery voltage I assume the alternator would put out more amps...or maybe not...maybe the 14.1V output is only at start up. Anyway, again...thanks for all the help! Cheers!

 

[RobNE]

Eldough,
I have a 2008 Tacoma with an eagle. I run a Hellroaring battery isolator and haven't had any issues (that i've noticed). My campers not on the truck, so i can't give you specific voltages, but while traveling I run the factory fridge (norcold?) off of DC (not sure of the current draw, but it will easily send a battery to nothing in 1/2 a day) and switch to propane once stopped. At night everything else runs off of batteries. So while moving i'm simultaneously charging and powering the fridge.

We've noticed no problems at all with our system. The hellroaring is supposed to be a very good system and has virtually no drop across it. I run 10ga wire to the camper, so there is some drop there. One of these days i'll install a meter so I can monitor battery voltage more accurately. The hellroaring needs no attention and was simple to install.

Rob

So I ran another test. Got the camper battery down to 12.44V and the truck battery rested down to 12.83V. With the batteries disconnected, started the truck and measured 14.18V at the truck battery immediately after starting. Let the truck run until the truck battery was down to 13.84V...about 15 minutes. Connected the batteries, again without the surepower. Measured 13.81V at the truck battery and 12.48V at the camper battery...which was slowly starting to rise. So I decided to put more of a load on the truck battery to see if I could get the alternator to put out more amps...I turned on the headlights, hazards, blasted the radio and AC. After 10 minutes, the truck battery was down to 13.44V and the camper battery was no longer creeping up. I disconnected the camper battery and measure the volts just before the camper battery = 13.11V...looks like a 0.3V drop when all the stuff is on. From this information, it sounds like my alternator is only putting out enough amps to keep the truck battery above a certain voltage. I was hoping to see 14.1V at the truck battery when all the "stuff" was on, but it didn't get there...basically stayed around 13.44V while I was observing...at some truck battery voltage I assume the alternator would put out more amps...or maybe not...maybe the 14.1V output is only at start up. Anyway, again...thanks for all the help! Cheers!

 

[HERR42]

i would suspect that the optima is tired. it is drawing too much current and is not charging properly.
swap the two batteries and re run the test.

 

[brett13]

Instead of starting a new thread, I'll just ask here:

So, I'm back from a few days out in UT and can't remember what voltage is a fully charged battery? After 6 hrs of driving, my camper batt is at only 12.8v. After one night of furnace use (it got down to the mid-20s), it was down to 12.05v. I thought it had more capacity than that. So,
1) what voltage is a fully charged (good) battery? Should it be higher than 12.8?
2) did the furnace really use that much in one night (keeping the temp in the low 50s from outside temps in the mid 20s?)
3) how long do these batteries last?
4) what voltage is "low"?

Thanks!

 

[pods8]

Instead of starting a new thread, I'll just ask here:

So, I'm back from a few days out in UT and can't remember what voltage is a fully charged battery? After 6 hrs of driving, my camper batt is at only 12.8v. After one night of furnace use (it got down to the mid-20s), it was down to 12.05v. I thought it had more capacity than that. So,
1) what voltage is a fully charged (good) battery? Should it be higher than 12.8?
2) did the furnace really use that much in one night (keeping the temp in the low 50s from outside temps in the mid 20s?)
3) how long do these batteries last?
4) what voltage is "low"?

Thanks!


1)If I recall right most major batteries sit 12.8-13.1V fully charged. My AGM's sit at 13.1V when fully charged.

2) Depends on how much your furnace pulls and how often, sound silly but really only you can answer that. Take whatever amps the furnance pulls multiplied by how many hours it ran and that is what you pulled. However over times batteries can loose capacity and even though they charge to 12.8V, for instance, they'll drain much faster.

3)Depends on the battery and how it was maintained. Charge it properly and it lives longer. If its a flooded cell that you maintain the fluid levels on properly it lives longer, etc. The more frequently you discharge past 50% the quicker you kill it, etc.

Rough guideline.
Marine: 1-6 years
Golf cart: 2-7 years
AGM deep cycle: 4-7 years

4) Might want to browse this whole link in general but here's the jump to the area about voltages verse current charge (can't format the chart here easily so the link will make the numbers below make more sense).
http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Voltages
State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

 

[HERR42]

Instead of starting a new thread, I'll just ask here:

So, I'm back from a few days out in UT and can't remember what voltage is a fully charged battery? After 6 hrs of driving, my camper batt is at only 12.8v. After one night of furnace use (it got down to the mid-20s), it was down to 12.05v. I thought it had more capacity than that. So,
1) what voltage is a fully charged (good) battery? Should it be higher than 12.8?
2) did the furnace really use that much in one night (keeping the temp in the low 50s from outside temps in the mid 20s?)
3) how long do these batteries last?
4) what voltage is "low"?

Thanks!



1. 12.8 is correct resting voltage for a battery that is at full charge. Immediatly after charging a battery will have what is referred to as "surface charge" and will read higher...for a while...until the surface charge dissipates.

2. that seems about right.

3. that my friend is a very good question. i attached a excel spreadsheet to model power consumption. see notes below.

4. that too is subjective. for me, this is when the battery voltage hits the upper threshold of the CO sensor's low voltage alarm. it makes the sensor chirp...and usually happens after a night of running the heater and laptop. When the heater starts it causes the battery voltage to spike below 11.6 volts and this causes a quick chirp. this begins during the early morning hours... 5am or so in cold weather. The heater and lights continue to work fine and will continue to do so as the battery voltage declines. The sensor alarms bother me, so if the sun is not available the following day I start the truck and charge the battery.



spreadsheet explanation:

This spreadsheet shows the power consumption of the various devices in the camper as measured with a Watts UP power meter made by RC Electronics.
I attached the meter in series at the battery and operated the devices singly while recording the meter readings.





Notice that the solar panel, a stock 50 watt unit, is a current supply, and notice that it only produces 6 hours a day. Shorten the number of hours and watch the effect.



Also listed, as a power source, is the power from the truck charging system during a single activation of my remote starter. Remote start is a feature of the truck’s security system and is activated by a button on the key fob. One run interval is 12 minutes. As you can see, one shot yields about 250 watts of charge current. No quantity of run interval is listed in the sheet so it’s just an FYI.


As shown, the overall consumption yields a small surplus of .546 amp hour in this model. obviously this would be a typical warm weather scenario with little heater run time, etc. ...and with the fridge on propane.

My experience shows that if the solar panel is shaded we really miss that charge time and we notice the discharge of the battery. We notice this when camped in high trees and dark skies. Such as Jed Smith state park in December.

My solution is to trigger the truck’s remote start 3 or 4 times in the evening.

 

[brett13]

Thanks.

Playing with the spreadsheet, if my furnace was on for 12hrs and running about 15min per hour, that is about 12.5 AH. Include LED lights and detectors and we probably used 15-16 AHs. The battery reads 75 AH, so it seems that the voltage reading of 12.05 (indicating 50%) is low considering we only used about 20% of its AH capacity, no?

I never really counted how long the furnace actually runs, so the above is a guess. Any ideas on actual run time when trying to keep the camper heated by say 25*F?

 

[pods8]

1. 12.8 is correct resting voltage for a battery that is at full charge. Immediatly after charging a battery will have what is referred to as "surface charge" and will read higher...for a while...until the surface charge dissipates.


The surface charge aspect is accurate for all batteries. However some do have a higher fully charged voltage, I believe 12.8-13.1V is the general range. As mentioned my AGM's do rest at 13.05-13.1V fully charged (this verifiable after the surface charge has had days to dissipate after the charger is turned off).

Thanks.

Playing with the spreadsheet, if my furnace was on for 12hrs and running about 15min per hour, that is about 12.5 AH. Include LED lights and detectors and we probably used 15-16 AHs. The battery reads 75 AH, so it seems that the voltage reading of 12.05 (indicating 50%) is low considering we only used about 20% of its AH capacity, no?

I never really counted how long the furnace actually runs, so the above is a guess. Any ideas on actual run time when trying to keep the camper heated by say 25*F?


If your usage is accurate (and you have the same furnace, thus similar amp draw) then it would be pointing to reduced battery capacity, which will happen over time. It will be amplified by various factors (poor charging, repeatedly draining the battery very low, etc.)

However on the flip side its quite possible you're underestimating the run time as well. For instances if you were running 30min/hr now you're right about the correct 50% usage for your battery in good health(keep in mind in cool weather they'll have a lower voltage for the same charge).

I know cold/windy weather can really pull some heat out of the rigs at times and I was surprised by heater usage. If you have some drafts in the doors, turnbuckle latches, had the vents opened up, etc. it can all add up quick to heat loss.

 

[HERR42]

a 75 amp hour battery will deliver 3.75 amps for 20 hours - and be discharged down to 10.5 volts.

here is the sheet on the interstate battery i got with my hawk.

look at the current draw listed to bring the battery to 11.1 volts in 10 hours, 6.7 amps per hour.

 

[brett13]

a 75 amp hour battery will deliver 3.75 amps for 20 hours - and be discharged down to 10.5 volts.

here is the sheet on the interstate battery i got with my hawk.

look at the current draw listed to bring the battery to 11.1 volts in 10 hours, 6.7 amps per hour.

If the furnace draws 4.15amps while running (according to the spreadsheet), then 4.15 x .25 (15min/hr) x 12 hrs = 12.45 AH. That should be worth less than 20% and correlate to a voltage of 12.42, but I was at 12.05. What am I missing or doing wrong?

 

[pods8]

If the furnace draws 4.15amps while running (according to the spreadsheet), then 4.15 x .25 (15min/hr) x 12 hrs = 12.45 AH. That should be worth less than 20% and correlate to a voltage of 12.42, but I was at 12.05. What am I missing or doing wrong?


Do you have the same furnace? If not your draw could be different.

You even said you didn't know if you were only running 15min/hr or not, if you happened to be running more like 30min/hr then its more accurate towards the 12.05V on a 75ah battery.

Temperature: The colder your battery gets the lower your voltage goes even if you haven't drawn any charge off it and it'll increase back up when the battery is warmed.

Battery Life: The older you battery gets the faster it will discharge. So if you have a battery that has lost 50% of its capacity, if you start with 12.8V on a 75ah battery then you're only going to get ~19ah on it before you hit 12.06V.

Lots of variables. So if you want answers you need to ask yourself: What do I really think my draws are (both quanity and time), how old is my battery and what temp was I pulling the temperature readings, how have I been treating my battery (lots of heavy discharges?), etc?