help understanding battery chargers

Re: help understanding battery chargers

What size (Amp*Hour / Voltage) and type (flooded cell, AGM, brand/model) of battery do you have?

Have you accurately measured the voltage at the battery terminal? Do you know the current flow into the battery bank over time and how much current (amp*Hours) you use?

And what gauge wire/size of battery charger do you have now.

Normally, you can force C/8 (battery 20 Hour rating / 8) continuous charge into a Flooded Cell type battery bank--and even more with some AGMs.

Re: help understanding battery chargers

Do you know the "group number" or "BCA" or some other battery sizing standard?

I will give some generic advise--but without knowing the details, it is going to be difficult to give better answers.

What size (Amp*Hour / Voltage)Dont Know/12V and type (flooded cell, AGM, brand/model) of battery do you have? wet cell

Lets assume this is a 100 Amp*Hour 12 volt battery (basically the size of a large automotive or Pickup battery).

The "high side" charging rate should be C/8:

• 100 AH * 1/8 charge rate = 12.5 amps

It will take ~4-6 hours to fully charge the battery that is 50% full

However, you can probably get it to ~85% charged by pumping current in at a high charging voltage (~14.5 volts) at the battery in a reasonable amount of time (2-4 hours). Leave the last 10+% for recharging when you get back home or a trailer park where you can plug in.

Have you accurately measured the voltage at the battery terminal? NO, How do I do that?

Get a Digital Multi-Meter that can read to at least 12.34 volts (3 1/2 digit meter). A more accurate meter would be nice.

Set the meter to 20 volt scale and measure the battery charging voltage...

Automotive battery chargers can be all over the place (some have different charging modes too)--so measuring the battery charging voltage will tell you if your automotive charger is doing well or not.

Note, when the battery is below 80-90% state of charge, the battery bank will be less than the charger output voltage. As the battery fills up, the voltage will rise to the charger's "set point" and will (should) hold steady until the battery is nearly 100% charged...

So, you may see 13.2 to 13.6 to 13.8... 14.4 to 14.5 volts, then hold at 14.5 volts for the next 2-4 hours or so... At that point the battery should be fully charged.

Here is a quick link on how to use a DMM (volts/amps/resistance)

As always, be careful with DMM's... Make sure they are set for voltage when measuring the battery voltage... If set for current when measuring voltage, you may be purchasing a new fuse/meter and underwear.;)

Do you know the current flow into the battery bank over time and how much current (amp*Hours) you use? NO I think this is my question but I'm not sure

There are several ways of approaching this problem... The "best way" is to use a Battery Monitor. The Trimetric meter is would be a good starting point.

You place a current shunt (precision resistor) in the battery negative lead and connect the Battery monitor to measure battery voltage and current (voltage drop across shunt).

The meter gives both "instant readings" (i.e., 12.7 volts and -3.0 amp current flow) and cumulative current flow (i.e., The battery is now at 95 AH capacity out of 100 AH)... Think if a fuel tank meter that estimates the gallons in the fuel tank by monitoring the fuel flow in and out of the fuel line. Fairly accurate--but they do need to be reset to 100% full once in a while (basically, battery is fully charged for a couple hours).

You will use the meter to monitor both how much power you pull from the battery bank (discharging to some where between 50% to 20% state of charge--never below 20%, and below 50% does reduce battery life over time).

And what gauge wire/size of battery charger do you have now.No idea, Whatever came on the travel trailer from the factory/10/8/2 amp charger

12 volt battery systems need a fair amount of current compared to 120 VAC systems... For example, a 12 watt filament reading light will draw:

• P=I*V
• I=Power/Volts= 12 watts / 120 VAC = 0.1 amps AC
• I=Power/Volts= 12 watts / 12 VDC = 1.0 amps DC

So, depending on your loads, wire gauge, and length--too much current creates a lot of voltage drop on the wiring... 1 volt drop is about as much as a typical 12 volt circuit can tolerate. Where as 1 volt drop on a 120 VAC circuit is virtually nothing (nominal maximum voltage drop on a 120 VAC circuit ~3.6 volts maximum).

And, this affects the battery charger/battery converter in the RV too... If you have a 0.5 volt from the charger to the battery bank--that really "hot" 14.5 volts for quick charging the battery bank becomes a very gentle 14.0 volts (at max current)--As the current falls, the voltage at the battery bank does slowly rise--so eventually the battery does become full--but this can add many hours to the charging time (fine if you are in an RV park, a killer using a gasoline genset).

Again, a great place to use your new DMM... With the battery discharged to less than 80% state of charge--Turn on the charger and measure the voltage at the battery and the charger--and see how much voltage drop you will have.

You may end up wanting to replace that 5' of 14 AWG wire (or whatever they used) with 8 or 6 AWG to reduce voltage drop at the battery when charging.

Normally, you can force C/8 (battery 20 Hour rating /
8 ) continuous charge into a Flooded Cell type battery bank I DONT HAVE ANY IDEA WHAT ANY PART OF THIS SENTENCE MEANS

We really need to know the approximate capacity of your battery bank and charge controller... If this is a small RV with a single 12 volt battery--you may have as small as a 100 AH or less battery in there (it could be 200 AH or more--I just don't know).

Remembering our rules of thumb that we aim for between 5% and 13% rate of charge ( C/20 to ~C/8 )--This is usually the most cost effective charging current...

Below 5%, it takes forever for batteries to charge, and the batteries may not last as long (various reasons).

Above 13%, the batteries can overheat and if done with solar panels--that is a huge amount of expensive solar array and charge controllers... For an AC battery charger you can probably go to C/4 (25% of Capacity)--But some of that may be wasted--Normally check recommended battery specifications... For generator charging with an adjustable charger, you could probably charge at 15.0-15.5 volts--but you need to turn off/crank down the voltage (to float) to prevent battery damage (when charging current drops).

Note that 14.2-14.5 volt charging is compatible with most all "12 VDC" appliances/electronics... At 15.0+ volts, it has been reported that some 12 volt devices may die (12 volt car adapters for computers, etc.).

If you have access to the battery cells--you probably want to get a hydrometer to measure your battery's specific gravity to check for state of charge/cell balance.

Read about batteries here:

Battery FAQ
www.batteryfaq.org

Now--how to put all of that together... Say that you don't currently have solar (or not very much) and need to use the genset for much of your charging--Noise, cost, and amount of fuel storage is a big issue... So, here is how I would proceed. Note I am making lots of assumptions that are probably wrong--so you will need to use your real information:

Assume 100 AH battery bank, and ~C/4 approximate max charging ( over C/8 ) to quickly charge your battery bank:

• 100 AH * 1/4 = 25 Amps maximum
• 100 AH * 1/8 = 12.5 Amps minimum

So, we are looking at:

• Xantrex Truecharge2 12-Volt, 20-Amp Battery Charger
• Iota DLS-15: 12 Volt 15 Amp Battery Charger
• Iota DLS-30-M 12 volt 30 amp regulated battery charger

The Xantrex TC2 is a very nice 14.4 volt charger. Has a remote battery temperature sensor (option: good for fast charging your battery and reduce chance of thermal run-away), is Power Factor Corrected (uses less genset current).

The Iota is much less expensive and is just a solid 14.2 volt charger (with manual float switch or IQ4 automatic 2 stage charge control--not really needed for a generator based charger--more for Grid Powered).

The 15/20 amp chargers will run on either a Honda eu1000i (900 watt) or eu2000i (1,600 watt).The 30 amp will probably need a eu2000i sized genset.

If your current chargers output appropriate voltage and current--probably no need to change them out right now.

Next, generator size... If you have a small battery / battery bank -- a 7,700 watt genset is absolutely going to bankrupt with fuel costs...

For most gasoline gensets, at 50% or below rated power, they will consume on the order of 50% fuel flow...

Assume that your genset uses ~1 gph at 100% load and ~0.5 gph at 1/2 load or less... That would mean that you are using ~0.5 gph for 4-6 hours or so to recharge your battery bank or 2-3 gallons of fuel (big SWAG).

A 20 amp charger on a small Honda eu1000i:

• 20 amps * 14.5 volts * 1/0.77 charging efficiency = 377 Watts

The eu1000i uses a minimum of 0.6 gallons of fuse at 1/4 load (900 watt rated) or 225 watt load for 8.3 hours (econo throttle on--reduce engine speed at low loads) and 3.8 hours per 0.6 gallons at 900 watts:

• 8.3 hours * 225 watts * 1/0.6 gallons = 3.1 kWH per gallon 225 watt or less load
• 3.8 hours * 900 watts * 1/0.6 gallons =5.7 kWH per gallon 900 watt load

So, somewhere better than 3.1 kWH per gallon. So 4-6 hours at ~377 watts:

• 377 Watts * 6 hours * 1/3.1 kWH per gallons = 0.73 gallons

With this setup--I would expect that you would use about 1/4 the amount of fuel using the small Honda eu2000i genset vs your 7.7 kW (unless you are running tools on a job site or something where you need to run the large genset anyway).

Now the above may be way overkill for your application--or may be well worth your time and effort (sort of the difference between camping 6 weekends a year vs living on a job site).

Problem is without knowing your power usage and battery bank size--I really can't do more. Perhaps a larger battery bank would be helpful (run the genset every 2nd to 3rd day). Or perhaps installing a 135 watt solar panel + charge controller could keep up with your power needs 9 months of the year (reading light, radio)...

You will be the best judge of what works well for your needs/lifestyle.

-Bill

Re: help understanding battery chargers

Bruce,

bgarrett wrote: »

I looked at my 12V batteries.
One has 'deep cycle' 31-950 850CCA 207RC. There are no other words or numbers on this battery. It is longer than a battery in a 1/2 ton chevy pickup

The other battery is a mack bulldog maintenance free with--- 180 min, repl pin bd31650A BCI group 31. Thats all the words and numbers on that battery it is longer than a pickup battery.

Those are probably Group 31 Batteries. The first one 850 Cold Cranking Amps and 207 Minute Reserve Capacity... Basically, these are automotive/truck starting batteries and most likely not deep cycle storage batteries.

Roughly, the storage capacity of a "typical" Group 31 battery is 95-125 Amp hours... So lets go with 100 AH.

The "problem" with using automotive batteries vs deep cycle (from Wind-Sun Battery FAQ):

• Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
• Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. This gives less surface area, thus less "instant" power like starting batteries need. Although these an be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge. Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf car battery is quite popular for small systems and RV's. The problem is that "golf car" refers to a size of battery (commonly called GC-2, or T-105), not the type or construction - so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.

These batteries are not going to last very long if you cycle them by more than 10-20% (90% to 80% state of charge).

For now, just use them until they do not meet your needs (next time, lookf for Deep Cycle, and don't get a Marine type battery either).

One is for lites at nite. one is for the shurflo pump for water out of the creek to bathe
I plan to repeat this living arrangement the next time the weather warms up.

Just use these two batteries to experiment and setup your system with... You can wait until they fail, or by a new set for spring.

Using separate batteries and charging them together is a bit tough... It would possibly be better if they where used in pairs (shared capacity meaning less battery management and/or deep cycling of one vs the other)--But for your current needs, it is not mandatory (normally, we try really hard to match sets of batteries so they "wear out" together).

For power use, as an example--say you use the ShurFlow pump for 20 minutes a day at 5 amps (that sounds like a long/cold shower):

• 5 amps * 20min/60min per hour = 1.66 Amp*Hours per day

A typical 100 AH battery you could take your time and cycle it to 80% of capacity--20% of a 100 AH battery would be 20 AH of usage before recharging (very "soft" usage):

• 20 AH / 1.66 AH per day = 12 days of use

The reason I am suggesting not to go below ~80% of capacity is storage batteries tend to sulfate if keep for days/weeks/months below 75% state of charge--If you recharge before you hit 75% state of charge, the battery should have a very long life (assuming otherwise well taken care of and good quality).

How much for the night lighting? Just a simple 12 watt filament bulb (dome light, brake light, etc.) draws ~1 amp from a 12 volt battery bank:

• Power = Voltage * Current
• Power / Voltage = Current = 12 watts / 12 volts = 1 amp

If you can replace the filament bulb with a 1-4 watt LED lamp--you would save quite a bit of power (just a small example).

You say 4-6 hours to charge, that sounds like what I have been doing and it seems unreasonable to run the generator that long
my charger has a 10 amp charge rate that they say is for starting a car. In my ignorance I have not used that setting, Think I should?
I spent 5 months last summer in the travel trailer in the woods. There was no "home' or trailer park to use to charge my battery so I spent $1000 for the 7700 watt gasoline generator hoping it would charge my batteries and operate my crackerbox welder

Hmm... This is where planning comes into play. To "quickly" charge the batteries you have... 200 AH at 12 volts... That would be a 20 to 40 amp battery charger (you need a large charger to pump current quickly into the battery bank).

Many small battery chargers for charging a car may have a 2 amp charging mode and a 10 amp jump start mode... The problem with using the 10 amp jump mode is many battery chargers will overheat if left running for hours.

Also, you need to check the charging voltage from the charger... You are looking for something that outputs around 14.2-14.8 volts or so. I have no idea what your charger is set for (or even if it has a "set point").

If you had Utility Power--if you had to leave the batteries plugged in for 24 hours to recharge the batteries, it would not be a big issue. In your case, because you want to limit the generator run-time--you really need as large as battery charger as can be supported by your battery bank (20-50 Amps).

When charging your battery bank, if you draw out, for example 50 AH, then to recharge that amount of current will take (50 AH / 200 AH = 1/4 of 200 AH bank capacity):

• 50 AH / 20 amp charger = 2.5 hours

But, the batteries as the go over 80-90% state of charge, they start accepting less current (absorb phase of charging--basically the constant voltage set point of ~14.5 volts) it takes around 1-6 hours (call it 2+ hours, lots of factors affect this final time) to finish charging to 100%...

So the total time to 100% charge for 50 AH recharge with a 20 amp charger:

• 2.5 hours + ~2 hours = 4.5 hours

You can monitor the current / voltage at the final stage of charging and cut it back to 1 hour when you are out camping using the genset--You have the battery mostly charged--so you are doing OK.

It is not worth the cost of fuel and even battery wear and tear to recharge to 100% capacity every day.

Break--continued on next post