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Old 24-05-2013, 08:18   #16
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Re: battery charging from my generator

You can also get a pretty accurate feel for SOC when charging by the battery bank current acceptance rate. If a 1100Ahr bank is accepting 100A, the SOC is low, if it is only accepting 10-20A, you are pretty close to float.

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Old 24-05-2013, 09:22   #17
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Re: battery charging from my generator

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Originally Posted by goboatingnow View Post
The battery determines that not the charger, you dont set absorption mode current, The charge acceptance of teh battery does that,.

And yes, the transistion from bulk to absorption is not abrupt so you could easily enter "absorption" at close to bulk currents.

OP I say , if you have no other method of charging, then you need to bring the bank to 100% or as close as possible certainly every couple of days or so, to avoid damaging the batteries. Hence once a week try and run the charger until you get to <10 A. otherwise I agree with others , stop around 20-30A

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This won't be my first stupid question, and I'm sure it's been asked & answered a hundred times, but here goes . . .

If a battery's internal resistance determines rate of charge, then why is it important for "smart" chargers -- whether the genset driven charger in this case or engine driven alternators -- to reduce their voltage during absorption & float modes? My layman's mind figures that, as long as the charging voltage remains higher than the battery's rate of resistance, then the battery will continue to accept some amps, albeit at a slower & slower rate as it approaches 100% SOC. I suppose this is how "dumb" chargers/alts. work -- for e.g. in our vehicles -- that is they're either on or off. The two AGM's in my diesel truck have lasted 8 yrs. thus far with this (OEM) set-up.

If this is the case, then does it follow that the size of one's charger/alt. will only influence how fast the batts. will charge up to 80%, i.e. during bulk mode? Hopefully this isn't a drift away from the OP's original questions.
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Old 24-05-2013, 09:35   #18
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Re: battery charging from my generator

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If a battery's internal resistance determines rate of charge, then why is it important for "smart" chargers -- whether the genset driven charger in this case or engine driven alternators -- to reduce their voltage during absorption & float modes? My layman's mind figures that, as long as the charging voltage remains higher than the battery's rate of resistance, then the battery will continue to accept some amps, albeit at a slower & slower rate as it approaches 100% SOC. I suppose this is how "dumb" chargers/alts. work -- for e.g. in our vehicles -- that is they're either on or off. The two AGM's in my diesel truck have lasted 8 yrs. thus far with this (OEM) set-up
To some extent youve answered your own question, chargers dont 'reduce' their voltage during absorption, they increase it. This is the difference between smart and dumb chargers. dumb chargers just set a compromise voltage and charge, as teh terminal voltage rises teh current slows and teh charger takes ever longer to finish charging.

Smart chargers detect when the end of bulk is approaching and then regulate to a higher voltage to hasten the absorption phase.

Quote:
If this is the case, then does it follow that the size of one's charger/alt. will only influence how fast the batts. will charge up to 80%, i.e. during bulk mode? Hopefully this isn't a drift away from the OP's original questions.
Yes correct, the size of teh charger in essence controls bulk phase. ( upto the charge acceptance value).

Quote:
I suppose this is how "dumb" chargers/alts. work -- for e.g. in our vehicles -- that is they're either on or off. The two AGM's in my diesel truck have lasted 8 yrs. thus far with this (OEM) set-u
Batteries in vehicles do very little work as, the alternator is powering the loads( thats its main function , not charging batteries) , hence they tend to stay near 100% which AGMs like and therefor give you long life, Boats have entirely different battery characteristics and AGMs are not a good fit for that application
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Old 24-05-2013, 09:37   #19
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Re: battery charging from my generator

Voltage equals current x resistance.

A charger simply supplies current at the rate it was designed for (say 100A).

When the battery resistance increases, the voltage will increase if the current is constant.

Too high of voltage and the battery fluid will boil.

So once a set voltage is reached based on the battery resistance, the charger starts to lower the current to keep the voltage constant. And the current goes down, and down, and down while the battery reaches full charge and high resistance.

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Old 24-05-2013, 09:58   #20
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Re: battery charging from my generator

Ample Power Company Home Page, go to the tech tab and download the Ample Power Primer.
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Old 24-05-2013, 10:09   #21
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Re: battery charging from my generator

so I left my generator run for awhile today , even ran the dishwasher. after about 3 hours the charger was down to around 23 amps: the next time I looked it was in float mode at 13.3 v. I definitely want to get a decent size.solar array . perhaps I will not have to run the generator at all
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Old 24-05-2013, 10:59   #22
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Re: battery charging from my generator

Hmm... I don't find the explanations about charge phases particularly clear today so here's my attempt:

1. bulk phase : constant amps, namely all the charger can give. The voltage will get higher and higher as the battery resistance increases, in order to keep the amps up. A 120A charger will charge at lower output when it gets hot etc. so don't be alarmed when you see 104A or so.

2. Absorption phase: constant voltage; when, during bulk phase, the voltage reaches a certain level, say 14.6V, this phase starts. The voltage can't go higher because then the battery would start boiling violently. So all we can do is keep it as high as this and see the amps go down more and more.

3. Float phase: after a timer runs out or even smarter calculations for top chargers, the float phase starts. The voltage gets lowered to say 13.2V so that the batteries can cool down while very slowly the charging will complete.

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Old 24-05-2013, 12:55   #23
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Re: battery charging from my generator

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Originally Posted by colemj View Post
....So once a set voltage is reached based on the battery resistance, the charger starts to lower the current to keep the voltage constant....
The charger doesn't lower the current, the battery resistance increases at it charges - this lowers the current.

Sometimes a picture helps:
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Old 24-05-2013, 14:00   #24
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Re: battery charging from my generator

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The charger doesn't lower the current, the battery resistance increases at it charges - this lowers the current.

Sometimes a picture helps:
The charge source does control the current and it is all based on the charge voltage it is set to. In bulk the charger is in CC or constant current mode, all it has, but once in absorption or float the charger then is in CV or constant voltage mode and adjusts the current needed by the battery bank to maintain, but not over shoot, that voltage setting. If the charger did not modulate the current the battery would over shoot the voltage limit. The battery is merely telling the charger, based on voltage, that it needs more or less current.

If the charger is set for 14.4V and the battery approaches this voltage with 100A of current behind it the charger will then need to begin reducing the current so as not to over shoot 14.4V..

You can turn the voltage up at that state of charge and more current will be produced by the charger, because the battery needs more current at 14.6V to maintain that voltage, or turn the voltage down and less current will be provided by the charger. If the charger ran CC all the time you'd have no way of limiting the voltage. All any charge source really does is "limit" voltage in CV mode. I like to explain their role to my customers as voltage limiters and it seems to make more sense to them.

In bulk the limit is the capability of the charger in current, hence constant current term most often used to describe bulk. Absorption is nothing more than a voltage limit that initiates the transition from CC to CV mode. Float is nothing more than another lower voltage limit. How & what each charger uses to execute it's move from absorption to float is different but float and absorption are nothing more than voltage limiting stages of charging.

It gets confusing because we hear all the time the battery "limits" current, and it does, but only in regards to not over shooting a set voltage. The charge source is really limiting or reducing current because the battery is telling it to by saying "if you keep throwing 100A at me I am going to 18V".... The charger watches the voltage and modulates the current accordingly. Often during a transition from absorption to float you will see no current supplied because if the charger just drops from 14.4v to 13.6V it may take some time for the batteries to naturally decay the surface charge to the float level. The charger in this case ceases all current until the battery voltage drops low enough that the battery needs some level of current to maintain the new and lower voltage limit.

As the state of charge rises and the internal resistance of the battery changes less and less current is needed to be supplied by the charger to prevent an overshooting of the voltage limit... Alternator regulators do the same thing by limiting the field to not over shoot the voltage set point.

This is very easy to see and monitor with an external regulator just put a clamp meter around the field wire and watch it in bulk. You'll likely see a steady 6A +/- depending upon regulator and alt configuration. As you hit the absorption voltage you can see the regulator begin to "limit" or reduce the field current so the alt produces less current and thus does not over shoot the regulators voltage setting it is trying to maintain.
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Old 24-05-2013, 14:22   #25
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Re: battery charging from my generator

Maine, et. al...

I know you know this, but the above is overly confusing.

The charger does not limit the current in CV mode. The battery does.

The charger limits its maximum voltage in the absorption and float stages....the battery determines how much current it will accept at that voltage.

Increase the voltage, you'll increase the amperage.

Hold the voltage steady, the battery will determine how much amperage it will accept from the charger....right up to the full capacity of the charger.

Example:

A battery bank is partially charged. It is accepting 100 amps @ 14.8VDC from a charger with a capacity of 120 amps.

Now, connect that same battery bank to a charger with a capacity of 500 amps. Set the voltage output of the charger to 14.8VDC. The battery will still accept 100 amps....no more.

The OP didn't tell us if those five 8-D batteries were flooded, AGM, or gel.
It would be helpful to know before we start making guesses about what his measured voltages show.

Bill
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Old 24-05-2013, 14:40   #26
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Maine Sail

Charger regulate voltage , if the load increases ( ie the effective load impedance falls ) the charger supplies more current. ( it output resistance is lowered by the regulation )

Conversely , as the load ( battery) equivalent resistance increases the charger regulates voltage by raising its output impedance and hence less current will be supplied

Hence the term is voltage regulation , in current regulation it's different.

Note at all times its the effective input resistance of the battery that determines the current

However modelling battery impedance is quite a complex model , hence all the simplistic descriptions.

The classic one is " in float mode the charger lowers the voltage ) you might ask yourself say how does a 50A charger control the terminal voltage of a 1000Ah bank !!!!

In reality all the charger does is increase its effective output resistance , effectively disconnecting itself from the battery , the characteristics of a lead acid chemistry the. Drops the terminal voltage down to the float voltage , the charger then reconnects as required to push a little current in ( by raising the voltage slightly )to " maintain " the float voltage.

Chargers are current sources nor sinks do the cant ' force ' the terminal voltage down

By and large , all voltages and currents are determined by the more powerful voltage source , ie the battery.

However we speak using conventions so well continue to say 'the charger lowers the voltage to the float level '

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Old 24-05-2013, 14:48   #27
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Btrayfors

You're right to a point. But LA is greedy and at higher voltages will accept currents that actually can cause its destruction.

So a very powerful charger is capable of driving the terminal voltage higher and destroying the bank.

LA has an input resistance characteristic that essentially within limits prevents runaway ( unlike say Li ) hence once the terminal voltage is maintained within a safe range the impedance will only allow a certain current to flow ( which we generalise as the charge acceptance rate )

Dave
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Old 24-05-2013, 14:58   #28
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Re: battery charging from my generator

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Btrayfors

You're right to a point. But LA is greedy and at higher voltages will accept currents that actually can cause its destruction.

So a very powerful charger is capable of driving the terminal voltage higher and destroying the bank.

LA has an input resistance characteristic that essentially within limits prevents runaway ( unlike say Li ) hence once the terminal voltage is maintained within a safe range the impedance will only allow a certain current to flow ( which we generalise as the charge acceptance rate )

Dave
Dave,

I said that batteries themselves limit the current at a given voltage.

So long as you limit the voltage to a given level, then ANY charger capacity won't be able to increase the amperage (current) more than any other........doesn't matter how large it is.

Of course, raising the charging voltage beyond the prescribed level will damage the battery. That's true for just about ANY size charger, not just for very big ones.

The key insofar as the charger is concerned is voltage control. The battery itself will take care of amperage control.

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Old 24-05-2013, 15:12   #29
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Dint say u were wrong bill.


Note that raising the voltage in itself will not damage the batteries,you have to supply a certain amount of current.

A voltage source ( charger ) supplies a load ( battery under charge ) determined by the impedance characteristics of the load.

No more then I can quite happily drive a LED from a 50 mA voltage regulated power supply as I can from a 1000A voltage regulated power supply ( using the same resistor )

Ie ie the V is fixed and the R is fixed then I is fixed. That's why batteries seemingly limit current

Dave
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Old 24-05-2013, 15:23   #30
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Re: battery charging from my generator

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Dint say u were wrong bill.


Note that raising the voltage in itself will not damage the batteries,you have to supply a certain amount of current.

A voltage source ( charger ) supplies a load ( battery under charge ) determined by the impedance characteristics of the load.

No more then I can quite happily drive a LED from a 50 mA voltage regulated power supply as I can from a 1000A voltage regulated power supply ( using the same resistor )

Ie ie the V is fixed and the R is fixed then I is fixed. That's why batteries seemingly limit current

Dave
Dave,

We're in complete agreement.....E=IR! Or, in this case, I=E/R.

Fixed voltage from the charger (e.g., 14.8VDC). Fixed internal resistance of the battery bank (e.g., 0.148 ohms). Fixed current or amperage (e.g., 100 amps).

It's a bit more complicated than that, of course, as the battery is really an RC circuit (inductive/capacitive/resistive) and we're really talking about impedences, not pure resistances. Still, the analogy holds.

Hold steady the voltage (the charger does this). The battery itself will determine how much amperage it will accept, and this depends on several factors, most important is it's state of charge and its chemistry.

Bill
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