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Old 31-08-2011, 06:18   #1
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Battery Charging with Load Applied = Incorrectly Charged Batteries

Unless a boat is winterised then I imagine that the normal situation is the batteries are charged with a load applied.
Thats the situation every day on my boat.... 4a at least 24 hrs a day.
The problem with this is that no charger is set up to handle this, resulting in overcharged batteries.

If you look around the internet and even on these forums you will see much info on correct bulk voltages 14.8 for wet cells, 14.4 for agms etc etc. So small voltage changes have a big effect in the charging world.

This exact voltage information always assumes that there is no load applied and even goes further to state that batteries must be rested after charging before checking their actual state of charge.

There never appears to be any importance on this fact... does anyone else view this as I do?
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Old 31-08-2011, 06:29   #2
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The only issue is the charger remaining in absorption mode too long. Bulk mode isn't a problem as the voltage is lower. Many smart chargers run a failsafe timer to exit absorption mode to address the issue. Most LAs can accommodate absorption voltages just below the gassing point for fairly long times. Temperature compensation is must.

But yes all battery chargers capable of significant current and voltages at or above the gassing point can damage batteries in certain circumstances.

Several chargers do mention they can be used as a power supply



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Old 31-08-2011, 06:34   #3
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Re: Battery charging with load applied = incorrectly charged batterys

I don't really understand what you are saying. Do you have a 4A continuous draw or a +4A continuous charge (4A above your usage)? What make and model is your battery charger?

A good 3 stage battery charger should keep your house bank charged without overcharging. That is its purpose.

So, short answer, I don't view this (and probably many things) the same way you do.
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Old 31-08-2011, 06:44   #4
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Re: Battery charging with load applied = incorrectly charged batterys

This is something for Bill Trayfors to weigh in on. I suspect that when batteries are receiving amps from alternate sources such as solar... or being drained of amps by applied loads... the voltage sensed at the battery will not be the same as a battery in a *rest* state and since smart regulators will establish the charging routine... ie amount of amps to try to shove into the batts, the voltage and so forth for the various *phases*. Smart regulators can and are fooled! I don't know how small loads will effect the program of the smart regulators... but I suspect there is some impact.

Apparently batteries will resist too many amps being forced at it and this is sensed by the smart regulators. If this is true.. perhaps someone such as Bill can explain how this works.

After years of trying to manage my electrical storage it remains a black box to me.
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Old 31-08-2011, 06:51   #5
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Re: Battery charging with load applied = incorrectly charged batterys

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I don't really understand what you are saying. Do you have a 4A continuous draw or a +4A continuous charge (4A above your usage)? What make and model is your battery charger?

A good 3 stage battery charger should keep your house bank charged without overcharging. That is its purpose.

So, short answer, I don't view this (and probably many things) the same way you do.
at least a 4 amp draw and normally more... could be an 8a draw with both fridges on. Victron 70a charger setup correctly.

The point is that all chargers calculate with the concept of no load applied (at rest)
so a battery with no load on charge at 14.8v is very different from a battery on charge at 14.8v with 8 amps of load from a running fridge and a running freezer and other devices. The voltage is not actually 14.8v as it is influenced by the amp draw of the running devices.
In this case it results in overcharging.... maybe not a problem for wet cells but for gels and agms this could be a disaster (lower voltages than 14.8v for gels and agms of course).
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Old 31-08-2011, 06:59   #6
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Re: Battery charging with load applied = incorrectly charged batterys

A load will not make much practical difference to battery charging. The regulation circuitry will simply raise the battery voltage to certain set points and hold the battery voltage for a set time. The main effect of the load current is that the battery charger has to supply the load current as well as the charging current, but it will do this automatically, unless the charging source is small in relation to load, but even then the countdown timer stops until the correct battery voltage is reached.
Sophisticated regulators fitted to some of the better solar systems also use the battery charging current as well as voltage to detect the charge endpoint. This is better, but these system are set up with shunts to distinguish the actual current entering the battery.
If you want to worry about battery charging I would be far more concerned that the charging voltages are often not adjustable on simple chargers and regulators.
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Old 31-08-2011, 07:02   #7
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Re: Battery charging with load applied = incorrectly charged batterys

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Originally Posted by Fuss View Post
at least a 4 amp draw and normally more... could be an 8a draw with both fridges on. Victron 70a charger setup correctly.

The point is that all chargers calculate with the concept of no load applied (at rest)
so a battery with no load on charge at 14.8v is very different from a battery on charge at 14.8v with 8 amps of load from a running fridge and a running freezer and other devices. The voltage is not actually 14.8v as it is influenced by the amp draw of the running devices.
In this case it results in overcharging.... maybe not a problem for wet cells but for gels and agms this could be a disaster (lower voltages than 14.8v for gels and agms of course).
The 8 amps will be supplied by the charger not the battery.
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Old 31-08-2011, 07:18   #8
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Re: Battery charging with load applied = incorrectly charged batterys

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The 8 amps will be supplied by the charger not the battery.
Yes thats true of my charger, when its in float mode, it does this as I have seen it....

but , what about absorbtion mode, for example when it might be putting in 30a, I then switch on some loads...
Does this influence the chargers ability to read the true at rest voltage of the battery?
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Old 31-08-2011, 07:35   #9
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Re: Battery charging with load applied = incorrectly charged batterys

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Yes thats true of my charger, when its in float mode, it does this as I have seen it....

but , what about absorbtion mode, for example when it might be putting in 30a, I then switch on some loads...
Does this influence the chargers ability to read the true at rest voltage of the battery?
The charger doesn’t read the rest voltage of the battery this requires no current in or out for a long time.
In absorption mode the charger will keep the battery voltage at the absorption voltage for a set time, say 2hrs at 14.2V.
When you connect a load the battery doesn’t know the load is attached, the current in and voltage remains the same. If the combined load and charging current is too great for the charger to keep the voltage at 14.2v the timer will stop counting down the 2hrs until the battery voltage rises once again to 14.2. When the total time the battery has spent at 14.2v is 2hours the charger will drop down to the float voltage.

Note the above ignores some minor effects from voltage drop etc. It also applies only to reasonably good chargers, cheap charges sometime have simpler charging algorithms, but these do a poor job of charging load or no load. The very best battery regulation uses current charging in relation to battery size to help make decisions about charging endpoints, but these will be fitted with shunts to measure this current in

Do not worry about the load, it will make little difference to the charging endpoint
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Old 31-08-2011, 08:31   #10
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Re: Battery charging with load applied = incorrectly charged batterys

Yes, Noelex is correct. With modern smart chargers, a modest load really doesn't affect the battery charging regime significantly.

Chargers like the Victron measure voltage and time in each mode, and these are settable values.

A good charger will not overcharge your batteries. By the way, charging voltages for AGMs and flooded batteries are nearly identical...and are, for practical purposes. It's the gels which must be treated a bit differently, by limiting the upper charging voltages.

There are several extremely knowledgeable battery experts who will tell you that batteries generally suffer from undercharging, not overcharging. I'm a firm believer in this, assuming you're using a smart charger and keeping an eye on the electrolyte levels.

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Old 31-08-2011, 12:26   #11
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Re: Battery Charging with Load Applied = Incorrectly Charged Batteries

I agree with Noelex and btrayfors but for slightly different reasons.

In reading the manual for by Newmar PT-25 charger, it appears that smart chargers are current sensing devices. As they detect loads, they supply current. In your case, the fridge comes on, presents a 4A load and the charger supplies 4A. No net charging of the batteries.

The 3-stage cycle seems to be triggered by turning the charger off/on. When switched "on" the charger senses battery current demand and supplies it up to its capacity. As the charge current demand decreases then the charger increases the voltage up the pre-set maximum. This is the Bulk phase.

The charger then enters the Absorption phase wherein it holds the voltage at the pre-set maximum and senses the battery charge current demand drop to zero (or some low level). Then it enters the Float phase and drops its voltage to prevent overcharging.

But, you say, "what if the fridge comes on and the current demand never drops to zero?" That's when the time out feature kicks in and forces the charger into Float phase after a predetermined time.

I am uncertain what would happen if a large load (say 90A in your case or 30A in mine) exceeding the charger current capacity is present for a long period of time and thus discharges the batteries. When the large load is removed, would the charger "reset" and begin a new charge cycle?
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Old 31-08-2011, 12:44   #12
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Re: Battery Charging with Load Applied = Incorrectly Charged Batteries

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I am uncertain what would happen if a large load (say 90A in your case or 30A in mine) exceeding the charger current capacity is present for a long period of time and thus discharges the batteries. When the large load is removed, would the charger "reset" and begin a new charge cycle?
There is a battery return voltage (usually with a delay). At this voltage the charger will start a new bulk phase.
This means if there is a net discharge for a reasonable time, the charger will recognize the battery has been discharged and start bulk charging again.
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