# charging a car battery

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#### Thunderchild

##### New Member
So I'm charging up a car battery with 2 solar panels (max 2.25 amps output from both)

now as far as I know at 14.5 volts 15 max the battery is charged, is this the voltage across the battery whilst under charge or when the charging powerr is disconected ? meaning do i have to disconect the panels and after a few seconds check the battery voltage or can I check it whilst under charge ?

if I were to charge the battery from a 14.5 volt regulator would this prevent the battery from overcharging ?

#### colin55

##### Well-Known Member
You need to know if it is a maintenance-free battery or a standard lead-acid battery as they have different final charging voltages.

#### MikeMl

##### Well-Known Member

The correct charging algorithm for Lead-Acid batteries :

1. Apply a fixed, well-regulated Vc. Initially, the battery voltage will start out low due to current limiting in the panels, and then slowly the battery voltage will rise to Vc. After reaching Vc, the battery charging current will begin to drop. That's when the regulator holds Vc.

2. Watch the charging current. When the charging current drops to less than 20% of the current that was flowing just as the battery voltage initially reached Vc, then switch the regulator to hold Vf (float voltage).

3. Hold Vf unless:
a) restart at step 1 if power is shut off and reapplied.
b) if a load exceeds the panel current and causes the battery terminal voltage to sag below Vd.
c) if the primary power is never interupted, about once a month go to Step 1. This prevents stratification of the acid, and is sort of an equalization cycle. This is more important for flooded-batteries than for sealed...

At room temperature:
For Flooded cell batteries: Vc=14.4V, Vf=13.2V, Vd=12.8V*
For Sealed batteries: Vc=14.7V, Vf=13.6V, Vd=13.0V*

*Note: these voltages need to be corrected for ambient temperature. I would also suggest that you look up the specs for the specific battery you are trying to charge. Most makers of sealed batteries have very specific voltages listed on their respective web sites.
A good reference is here.

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#### MikeMl

##### Well-Known Member
if I were to charge the battery from a 14.5 volt regulator would this prevent the battery from overcharging ?
No, this would cook the battery in less than a couple of weeks. See my other post.

#### tcmtech

##### Banned
Would this circuit possibly work for what you need.
It is a high/low window switch that puts a load on the system at a specific high voltage set point and continues to pull the voltage down with a load until a specific lower voltage set point is reached and then it turns off the load it will then repeat this cycle as often as needed to keep the peak voltage under control.

Perhaps having it turn on at say 14.4 volts (common automotive running voltage)and then turning off at around the float voltage point 13.4 volts. This would still give you the full battery charge capacity but wont actualy let it overcharge and cook itself to death.

But you would still be getting your extra energy back as heat or as whatever you use as an actual load bank like running Lights or an electric motor.

Being a 12 volt system the voltage regulator would probibly not be needed or it could be replaces with a lower voltage unit too. The actual circuit running voltage is not that critical.

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#### MikeMl

##### Well-Known Member
TCMtech, the goal for standby battery charging is to recharge at 14.4V, but float near 13.2V. Your circuit could do one or the other. It would take some additional smarts to switch between the two modes.

Continuously cycling a lead-acid battery from say 14.0V to 14.4V for 12 hours a day would dry it out pretty fast. Even cycling from 14.5 down to 13.2 and then repeating would mean that the average battery voltage is much higher than optimum. Even if there is a continuous load on the battery, the average voltage should be closer to 13.2.

The only reason cars get by with 14.2 to 14.8V while the engine is running is that it on average, the car runs less than two hours a day. Police cars and delivery vehicles typically have their voltage regulators set much lower...

#### colin55

##### Well-Known Member
Modern car batteries also incorporate "maintenance-free" technology.
The plates include a "metal" to increase the voltage at which "gassing" occurs. This means the charging voltage can be increased slightly before the "water is boiled out of the battery."
That's why modern car batteries, even though they appear to be ordinary "lead-acid" batteries, require very little "top-up."
You need to find out what type of cells you have, to be able to charge them correctly. It's very technical.

#### tcmtech

##### Banned
Ah... true, but the whole point of the adjustablity is to be able to set it where you want for what works best in that application.

There is theoreticaly perfect charging conditions and then there are real life applications.

Also the first sentence was would this possibly work, not this will theoreticaly be perfect.

I dont use theoretical consepts very much, I prefer real life application.
Theoretical says too many things wont work, real life says they work just fine.

In normal daily life usage most batteries are charged up to the maximum voltage and then put into use. Hmm.. isnt using a battery going to drain it down way below its float voltage too? But then when you recharge it are you not bringing it back up to the maximum voltage again?

I could be wrong but what I put in the first post sounds just like a real life charge discharge cycle to me! But under actual controled voltage limit conditions.

Using standard automotive starting type batteries for back up power tends to kill them before they ever dry out anyway. Plus that is why many good ones still have filler caps on them. SO a person can top them off as needed.

I personaly never buy batteries that I cant top off.
The perfect charging circuit is great but for what it cost I can put a lot of water in a battery too!

I just gave out one possible way of doing things instead of peeing all over him for not asking directly for a "super ultra multistage microprocessor monitored battery charger" system.

What the end user desides is utimately up to him.

#### nickelflippr

##### Member
This link may have been posted on another thread here, but is in tune with what MikeMi is saying. Charts are for a generic, and AGM battery charging algorithm.

#### Willbe

##### New Member
Minimum charging current is probably 4A or so. That's a pretty big solar panel.

#### Thunderchild

##### New Member
No, this would cook the battery in less than a couple of weeks. See my other post.
to be born in mind that at the moment the battery cannot fully charge in one day, it needs 2-7 days (weather dependant)to fully charge so in effect I'll always be at step one, Am I taking these measurements whilst the battery is under charge or by pausing the charge supply and seeing what voltage the battery has reached ?

#### tytower

##### Banned
My 50 years experience with lead acid batteries leads me to the following conclusions .

They are generally made for vehicles
They are made to take a big draw on start up and then recharge via an alternator very quickly
The modern ones last 3 years
Any continuous charging stuffs them pretty quickly with what appears to be sulphation
Charging input over 13,8 volts stuffs them
The plate connections are thin and easily break with a jarring
There is a continuous falling of lead compounds down into the bottom well over its working life.
The sulphuric acid clouds over their life
The negative terminal is always giving off electrons and connection to ground on vehicles increases the rusting of the body by electrolysis through moist air. Old Landrovers connected positive to ground are still around and going strong with little rust in chassis or steel panels

This makes some of the above posts look pretty bloody stupid

Ive been waiting for new battery technology to emerge for all the above years and NiMh seems great to me .Anything should be better than these pieces of junk but they work and do so in their trillions so who am I to complain.

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#### Willbe

##### New Member
A fully charged LA batt. with no surface charge should read 12.76 vdc.

#### Willbe

Hybrid car batteries cost about $20K which the dealership parts depts. will absolutely not tell you. #### tcmtech ##### Banned Most Helpful Member I'd rather not know how much 12 V of 100 AH NIMH costs Not as bad as you may think! Brand new around$2000 - $3000 Surplus new around$1000 - $1500 Used but good around$200 - $1000 LA standby batteries from industrial take out 12v 100 AH around scrap price!$15 - $35 core return when dead$5 - \$10

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