How to Monitor Battery Charge

Van Living Forum

Help Support Van Living Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

KennethCallahan

New member
Joined
May 29, 2016
Messages
1
Reaction score
0
So I'm at the tail end of a long camper van project, and I'm almost ready to hit the road. One of the last things I have to do is connect my 155 AH Vmax battery to my starting battery for charging, and I intend to do so with either a continuous duty solenoid or an isolator. 
My question is how to monitor the battery's state of charge so it doesn't drop below 50%. I'm not keen on dropping 200$ for a monitor that tells me the exact percentage, but I'm a little confused about a how volt meter would tell me what I need to know. Also, where all do I need to install fuses, and how large? Honestly all the electrical stuff is kind of confusing the everliving sh(arks) out of me. 
Thanks so much.
 
Place fuses close to the battery, ie; not more than 7 inches from the positive terminal. Size the fuse to protect the wire. ie; 14 AGW use a maximum 30 amp fuse. Then the fuse blows before the wire burns.
 
I'm pretty sure that's an AGM battery, so your methods of testing are Voltage and Load testers. Load tester: https://www.batterystuff.com/battery-products/battery-gauges-monitors-alarms-testers/EZB1100A.html

Read battery testing section: https://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html

Or a Trimetric monitor: http://www.solar-electric.com/bogar...iESzywBah02G_TKYZaTTVPJ4TV4vgwzepgxoCQWDw_wcB

I "think" you can load test a wet cell with a volt meter, but not so with a sealed battery: http://www.homepower.com/articles/s...ts/load-testing-batteries-without-load-tester
 
See if any of  THIS  helps!  
It's basically, what I'm doing in my minivan.  But.. I'm having a pro do it for me. :(   Cha Ching!

You could also read this  REALLY LONG  (and scary to me, a female non mechanic!) post made by a really knowledable fellow on here (SternWake) regarding my question and his opinions on the above link, which I found very helpful.

I'm also wondering about some sort of (easy!) way to check the batteries' charge.
 
Using voltage alone as an indicator of state of charge cannot be accurate with a battery still in use.  The battery needs time for voltage to stabilize before its voltage can equate to state of charge, for that particular battery.


These voltages will be different as the battery ages.

How fast the voltage rebounds after loads are removed can indicate battery health, but is really only comparable to that same battery when it was new.

How far the voltage sags under a particular load can indicate health of the battery, but only when there is a baseline to compare it to, measurements taken when the battery was new.

VOltage is a bit like throwing a rock at a taut rubber band and guessing the weight of the rock by how far the rubber band stretched.  Only a vague sense is possible.

However if one were to carefully balance the rock on the rubber band, and this process took hours, then one could form a reasonably close guess, if they also knew how the battery performed when new during the same test.

As far as when to recharge, if you can recharge, do so, anytime.

If deciding when to stop discharging, 12.2v is generally considered 50%, but this is with no load.  Put a 5 amp load on a 100AH battery at 50% and the voltage might hit 11.8, and when load is removed in 15 minutes it could be 12.1, in an hour at rest, voltage  it could be 12.15, and at 3 hours of no load voltage might rebound to 12.21v, and one can then say !Eureka!!!, I've hit 50% and can now recharge.

Or one could have seen that 11.85v reading while typing on their laptop with ventilation fan running and said OH EFF, I need to turn off loads or go below 50%, but perhaps I already am under 50%.  Either way, the battery needs to be charged.

Guessing when to stop dischrging via voltage alone can certainly be good enough, but far too many people think and want voltage to read a battery like an accurate  fuel gauge or % remaining like on their phone( which is usually wrong too), and this is simply not possible when the battery is being charged or discharged.

Amp hour counting monitors are great learning tools, but even they can be inaccurate if programmed incorrectly, and they drift out of reality with many accumulated cycles and need to be rezeroed when it is know the battery is indeed fully charged. Which in itself is not really possible to infer using voltage alone, much to the chagrin to those who want voltage to precisely equal a %, and perhaps refuse to believe otherwise.
 
Total accuracy is complicated, but ball park is pretty easy. Here's what you have to always remember:

1) A battery that is being charged--power going in--will read falsely high--the bigger the charge and fuller it is--the further off it is.
2) A battery that is being drawn from--power going out--will lead falsely low--the bigger the draw and lower it is--the further off it is.
3) A well rested battery--which means hours without any activity--nothing going in or out--is pretty darn accurate. The longer it's rested, the more accurate it is.

Two extreme examples:

1) A battery properly being charged at absorption voltage can read 14.8, but stop charging it and it wiill eventually return to reality which is still about 12.8
2) If my battery reads 13.2 when I turn on my microwave, the huge draw makes it very wrong, it might read 11.6-11.9. As soon as I turn it off it jumps right back to 12.8.

Assuming you sleep at night with everything turned off, in the morning before sun hits its it, the voltmeter is very accurarte.

Start watching it and learning how it works. Over time you can get a pretty good feel for what it really is by knowing what the volt meter says and what's been going on with it. For most of us, that will be accurate enough.

Especially look at it in the evening and then in the morning, you'll know how far off it is on a typical night.
Bob
 
A marine 12 volt guide I have says at rest like Bob described 12.8 is full, 12.6 is 3/4, and 12.4 is half. With a lead acid battery you can also get what is called a hydrometer. It measures the specific gravity of the acid inside of the battery. It is not exactly a handy tool to use, but occasionally by testing all the cells in the same battery you can see if you have a weak cell. The battery is only as powerful as it's weakest cell.

The best hydrometers are made of glass and have a thermometer built into them, (the reading will change with temperature). I use a plastic one that seems to be good enough. https://store.snapon.com/Hydrometers-Battery-Hydrometer-Blue-Point--P637143.aspx

Here are the items I used to link my batteries.
relay http://www.ebay.com/itm/401111987343?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
fuse holder two, one at each battery http://www.amazon.com/Bussmann-HHX-...&redirect=true&ref_=oh_aui_detailpage_o00_s00
fuse 60 amp http://www.amazon.com/Bussmann-MAX6...263_img_2?ie=UTF8&refRID=1JB3DD3J7EQKMVCMQEKH
Wire heavy enough to carry 60 amps. 6 Gauge would work. ( I had a bunch of 10 ga, so I put 4 of them together for each leg). I ran them through a scrap piece of garden hose.
 
A big plus to having a heavy battery interconnect is you can put a large inverter on the house battery and if you want to run something heavy like a microwave or hair dryer, you can start the vehicles engine and the alternator will help run the device.

If you were to run just a small wire to charge the house battery, you would not be able to run a large inverter for more than just a few minutes. (The house battery would drain and only a small amount of your vehicles alternator output could be put to use). A 14 Ga wire only carries 15 amps. It would also take much longer to charge the house battery from the vehicle's alternator. The battery might be willing to take a 40 amp charge, but if the wire feeding it can only carry 15, it will slow things down. The wire is also likely to get warm.
 
Please note that whatever the actual and particular math works out to be in the following example regarding copper cross section ....1 single wire of 4 awg would be superior to 4 parallel runs of 10 AWG, if 4 parallel runs of 10AWG would equal the same amount of copper 4 AWG. There are sites that will give the actual math as to how many parallel runs of XXawg in parallel......will equal one run of 4 awg................. I do not know off the top of my head

Also keep in mind that each of those four 10 AWG parallel runs should be fused individually at ~ 30 amps each, whereas one run of 4awg can be fused once at ~100 amps for less overall resistance.

Also note that much wire sold ,is rated as SAE gauge, which is significantly thinner than AWG, by 12 to 20% or so. Don't hold me to those exact numbers, just know 4 SAE gauge is noticeably thinner than 4 AWG, and marketers are born liars, and proud of it, so please do not believe their manipulations, or even product descriptions. Truth in advertising has never occurred and certainly hasn't tried recently. Anything but.

Some people like to get caught up in wire ampacity, or what it is rated to carry.

When there is a hungry battery at the end of a circuit, the thicker wire can allow much larger amperages to actually reach the battery, as voltage at the battery will be higher. Higher electrical pressure at both ends means higher amperage delivered. higher mperage delivered meas faster recharging. Faster recharging means that the next discharge begins at a higher state of charge. A higher average state of charge means higher battery longevity, and a higher to cycle per $$ ratio.

A higher cycle to $ ratio means more travelling budget available to you.

Thick copper is your friend when depleted batteries are involved, except when idling( not moving) to recharge, as it is possible the alternator might overheat or wear out prematurely from being maxed out and not having the ability to trnasfer the heat from itself via a fast belt driven spinning fan and vehicle speeds fast enough to flush hot air from underhood and keep alternator cooler.

**++**
take note some (lesser $$) AGm batteries will say to limit charging amps to no more than 30amps per 100AH of capacity. Thick copper and a fast spinning cold alternator can exceed this limit for long enough for that warning to become very valid, and in this instance thinner copper can safely limit the starving battery from overeating and purging from both ends.

Usually alternators are more expensive than batteries. There are exceptions. A hard working alternator will wear out faster, especially at low speeds when battery is depleted.
Idling to recharge is usually very hard on an alternator. Avoid unless temperature data reveals otherwise, but safe to say idling to recharge a depleted battery is very hard on an alternator.

hHgher roms move the alternator fan spins faster. Higher vehicle speeds keep the underhood temps lower, keeping alternator cooler, allowing into produce more juice, for longer.



Many people make the mistake of saying XX AWg wire can only pass so much current, or will always be able to pass this much current, without realizing that the voltage at the battery end of the circuit is a huge factor in how much amperage the battery can ask for at that voltage. When a battery is depleted below ~80% , thicker wire between charging source and battery ensures that the battery can reach 80% significantly faster, regardless of what any ampacity chart says is possible. There is much BS online concerning wire cross section and the ability to recharge a depleted battery, the BS being from those who read an Ampacity chart without really understanding OHMs law.

The battery can also easily take a high amp recharge. High amp recharges are not 'ideal' or necessary or required(usually) when one has all the time in the world needed to recharge. but when the next discharge cycle begins late afternoon, the high amp recharge is less detrimental, compared to to beginning the next discharge at a lower initial state of charge, because someone's grandpa once said that a slow trickle charge is always best, no matter what.

The trickle charge mentality should be forgotten, disparaged, respectfully and perhaps quietly with prejudice on a daily deeply cycled battery.
Grandpa was wrong in this instance.
No need to tell him though, Grandma's been doing that since day one, give him a break.

Thick wire between depleted battery and charging source, Will pay for itself in battery longevity.

Somebody just ran a triathalon and is gasping for breathe. You gonna say breathe through this kinked cocktail straw instead of this doubled Mcdonalds soda straw?

Do not do the same thing to your depleted battery while quoting an Ampacity chart that says the cocktail straw is perfectly adequate and safe.

This post was made for general consumption, not to disparage anyone. No offenses intended.

Ampacity charts have sent many a battery recharger down a path of mediocrity and premature battery failure.

Pay more for adequately thick copper between chargign source and depleted battery now, or pay much more for recycled lead later and more often.
Your choice.

As usual, it is much easier to do it right the first time, than correct the issue hoping to get it right the next time.

'Good enough' has its limits, and more so when it comes to recharging a lead acid battery, which are all stubborn selfish little ?@^(&WWW's which really want a stupidly intense recharge regimen to even approach 'ideal' cycle life.
 
Comparing 4 x 10AGW to 1 x 4AGW. End diameter measures area of 5.2612 mm squared for 10AGW. Times four wires is 21.045 mm squired. While 4AGW is 21.1506 mm squired. Insignificant difference. However the 4AGW only needs one connection at each end, less connections, lower loss.
 
10 gauge wire can carry 30 amps. So four 10 gauge wires can carry 120 amps. I fused them at 60 amps or half of what they could carry. Being all the same length of the same material, there is nothing that would cause more amperage to flow through one wire than the other. It is your basic parallel circuit with each branch having the same exact resistance. Both ends were crimped and soldered into copper connectors that fit the battery and relay terminal.
Wire is made up of individual strands I see no difference if those strands are grouped into four bundles or one. The cross section is the same in both cases. Had I stripped the insulation off of the wires put them together and re insulated them would that give a different cross section?
 
DannyB1954 said:
10 gauge wire can carry 30 amps. So four 10 gauge wires can carry 120 amps. I fused them at 60 amps or half of what they could carry. Being all the same length of the same material, there is nothing that would cause more amperage to flow through one wire than the other. It is your basic parallel circuit with each branch having the same exact resistance. Both ends were crimped and soldered into copper connectors that fit the battery and relay terminal.
Wire is made up of individual strands I see no difference if those strands are grouped into four bundles or one. The cross section is the same in both cases. Had I stripped the insulation off of the wires put them together and re insulated them would that give a different cross section?

DannyB 
That was exactly my thinking .
Watch my YouTube video of how I wired my Honda generator using mtripple
 
I wasn't offended, I just thought if you didn't understand my block diagram, it would be difficult to explain it.
Basically the inverter and battery act as a shock absorber for current draws that exceed what the generator can put out. If the air conditioner comes on and draws 1,500 watts for a few seconds the energy in excess of what the generator puts out will come out of the battery. When the A/C shuts down, the generator / charger can put the energy back into the battery.

I wanted a 800 watt generator because it is small, light, quiet, and runs most of the day on a gallon of gas. It isn't big enough to power a lot of things, but then most of those things don't use power continuously, they turn on and off, or like a microwave only run for short periods. If I am not running anything heavy, I don't need to run the generator, (or run it an hour a day to charge the battery). If I go into town, the car's alternator can charge the battery. I may even attach a 200 watt solar panel to the system but for the days I need a lot of energy the generator will be the go to. I am not a full timer, so I don't think large solar panels are my best option. Even if you have the room for them, what it would take to do the same job as a small generator could cost thousands.
 
Dakota Camper17.jpg DSC01568.JPG

Here is the mount I made for the house battery and generator. The charging system from the truck's alternator is finished. Today I may install the inverter and charger inside of the cab of the truck. I may even try my hand at bending the wood for the roof bows.
 

Attachments

  • DSC01568.JPG
    DSC01568.JPG
    1.8 MB · Views: 15
  • Dakota Camper17.jpg
    Dakota Camper17.jpg
    404.1 KB · Views: 5
nice mount. I would enclose the front and back to protect from road debris. the paint on my service bed is beat to hell where that generator is. highdesertranger
 
I have some thin sheet metal from computer cases, Or maybe just screw on some plywood treated with stain.
 
Top