Charging deep cycle batteries under load

slyttle · February 2013

Hi There,

Appologies if this is a newbe question, but I didn't see any threads specically on this topic.

Most chargers I have seen feature multi-stage 'smart' charging, that have bulk loading, float, and conditioning cycles.

The trouble I am running into is that my batteries (deep cycle FLA type) are continuously loaded. The charger appears to be able to handle the loads and keep the batteries charged. However no matter what, any load will always throw off the voltages and confuse the smarts within the charger.

This issue is highlighted when the charger goes into bulk loading mode and never is able to achieve the 14.3 Volts it needs to switch into a conditioning or float cycle. I end up disconnecting the battery from the loads in order to allow the charger bring the voltage up to the desired level, to switch back to a float status.

What's also concerning is that when the batteries are being maintained while under load, my inverter reads 12.65 volts rather than the expected 13.3 volts for float charging. I suspect everything is ok, because the voltage remains stead at 12.65 volts for as long as the charger is in foat mode ...ie. the system reaches a steady state.

I just don't know whether I am hurting my batteries, and how to get around the confusion that my charger encounters due to the coninuous loads.

Any advise you can offer would be appreciated.

-Sheldon

NorthGuy · February 2013

Re: Charging deep cycle batteries under load

For the battery to be charged under load, the charger must provide charging current plus the current necessary to run the load. From what you describe, my guess is that your charger is simply too small for this.

vtmaps · February 2013

Re: Charging deep cycle batteries under load

Welcome to the forum.

More info needed. What model battery charger? What type of load? What battery? --vtMaps

Cariboocoot · February 2013

Re: Charging deep cycle batteries under load

*Put on broken record*
Not enough panels for the battery bank ....
*Take off broken record*
... and loads.

You're suffer from net vs. gross charge current syndrome.

slyttle · February 2013

Re: Charging deep cycle batteries under load

I am starting out REALLY small so can experiment before I move forward.

So right now I just have two Deep Cycle 100 Ah 12V flooded lead acid batteries in parallel. The chargers is one that I had kicking around from my boat .... a BassPro 20A dual bank marine charger (10A per bank).

Even though I only have one 'bank', I simply hook the leads for one bank onto the first battery, and the leads for the second bank onto the second battery .... it is all hooked up in parallel so I would guess that one re-enforces the other and I should get 20amps of charging ... Correct?

Given that I have a total of 200 Ah of potential storage, I figured that 20 Amp from the charger should be ample (10% rule of thumb)

My naive though would be that no matter how big a charger I have, the voltage will always get skewed by the continuous loads (almost like inserting a battery that never charges into the curcuit) ... So having a continuous load would always throw off a staged charger that is only expecting unloaded batteries in the circuit. Am I way off here?

BB. · February 2013

Re: Charging deep cycle batteries under load

What is the size/configuration of your solar array?

Bill

Cariboocoot · February 2013

Re: Charging deep cycle batteries under load

Yes, but: the 20 Amps is gross. How much are the loads drawing? That current gets subtracted from the gross to give you the net charge current. The net rate really needs to be above 10 Amps to keep charging going while the loads are running.

Then back to the panel issue. What have you got for panels? Just because the controller is capable of 20 Amps doesn't mean the array is. This is probably an PWM type controller, so your panel Imp has to add up to 20 Amps + in order to maximize the charge rate. Rough estimate: more than 350 Watts.

NorthGuy · February 2013

Re: Charging deep cycle batteries under load

slyttle wrote: »

My naive though would be that no matter how big a charger I have, the voltage will always get skewed by the continuous loads (almost like inserting battery that never charges into the curcuit) ... So having a continuous load would always through off a staged charger that is only expecting unloaded batteries in the circuit. Am I way off here?

Almost everyone with solar setup has loads powered while charging. How big is your load?

slyttle · February 2013

Re: Charging deep cycle batteries under load

Right now it is just powering a computer rack ... (modem, a couple of routers, a switch, and a media server) ... all told, it wouldn't amount to more than 300W at the most.

Cariboocoot · February 2013

Re: Charging deep cycle batteries under load

slyttle wrote: »

Right now it is just powering a computer rack ... (modem, a couple of routers, a switch, and a media server) ... all told, it wouldn't amount to more than 300W at the most.

300 Watts / 12 Volts = 25 Amps.

See what I mean?

ChrisOlson · February 2013

Re: Charging deep cycle batteries under load

slyttle wrote: »

So having a continuous load would always throw off a staged charger that is only expecting unloaded batteries in the circuit. Am I way off here?

That's not totally correct. There are few, if any, off-grid systems that never have a load on the battery bank. And "staged" chargers don't care whether or not the bank is loaded.

In order for the charging sources (solar, wind, hydro, generator) to properly charge the bank, the charging sources have to supply all the power to meet the loads, PLUS at least C/10 current rate to charge the batteries during the bulk stage.

You have barely enough to meet the C/10 requirement of the bank. You don't have enough to meet loads plus charging requirements. Once you add more charging power, the "smart" charger won't be confused because it will maintain the correct charging voltages with the system under load.

This is no different than others have already stated here - just that your assumption about a continuous load on the bank throwing off a "smart" charger is not accurate.
--
Chris

slyttle · February 2013

Re: Charging deep cycle batteries under load

Darn that Ohm's law!

Ok ... I get you .... Looks like I'll have to go looking for a bigger, better charger

thanks for the lesson

inetdog · February 2013

Re: Charging deep cycle batteries under load

slyttle wrote: »

My naive though would be that no matter how big a charger I have, the voltage will always get skewed by the continuous loads (almost like inserting a battery that never charges into the curcuit) ... So having a continuous load would always throw off a staged charger that is only expecting unloaded batteries in the circuit. Am I way off here?

Yes, you are.

If the load current is greater than the maximum current from the charger, then the voltage seen by the charger will be lower than it expects, but since the battery is not being charged anyway it does not matter.
If the load current is less than the maximum current from the charger, the battery will not be delivering any current to the load and so the voltage seen by the charger will be the same as for a battery without load.
The one point that may be affected will be the transition from Absorb to Float. If the CC makes that shift by looking at the current to the battery, and does not get that current reading from a shunt which is in series with the battery, but instead goes by current delivered by the CC itself, it will will not see that the Absorb current has dropped below the End Amps setting and will have to go entirely by time elapsed to limit the Absorb stage.
But for a solar PV power source, it is relatively unlikely that the CC will get to the end of Absorb before the sun runs out anyway.

Blackcherry04 · February 2013

Re: Charging deep cycle batteries under load

slyttle wrote: »

Darn that Ohm's law!

Ok ... I get you .... Looks like I'll have to go looking for a bigger, better charger

thanks for the lesson

If your going to do what you doing a Power Converter / Battery Charger might be a better option. The IOTA with IQ/4 for Instance will not trigger a Bulk cycle until the battery voltage drops below 12.5 volts. On the other hand if you unplug the IQ4 it will. Just because the float voltage cannot be maintained, it still has to have a trigger and some won't automatically go to a higher voltage. 20 amps may not be enough, that's a different issue. A Magnum Inverter / Charger will not re-trigger until it under 12.5 Volts.

Blackcherry04 · February 2013

Re: Charging deep cycle batteries under load

Here is a response from Xantrex that was posted in another thread, it's applicable to the subject here. Not every charging source will be exactly as this, but they all kind of follow the same logic. Some Charging sources have a very low limit on the amps they produce while in Float Mode. I get this all the time, This is the reason some chargers are not acceptable if you cannot trigger a new bulk without some work around, it doesn't matter if they are a $20 charger or a $700 charger, you just end up wasting gas and that not cool on a 20 KW generator @ $10.00 a hour run time.

Posted here: http://forum.solar-electric.com/showthread.php?18957-Custom-Charging-settings-for-my-AGM-batteries-let-me-know-if-I-m-still-too-dim

Max Current (%C)
The maximum available charging current (in A) as a percentage of battery
capacity (in Ah) for the Float charging stage.

During float mode, the charger is in Constant Voltage mode(13.4V default) and in a simple system the battery's acceptance of charge current (State of Charge) is actually what determines how much float current is being absorbed into the battery.
A 200Ah battery bank in good health will usually have a float current well under 10A so setting the charger's Max (output) Current %C setting to 200%C will not force excess current in to the battery. In this scenario, the charger will be idling along in 13.4V float operation at <10% of it's amp capacity.
Now, introduce a typical boat installation where there are other potentially high current 12V loads, boat winch, bow thruster etc which could easily demand an extra 30-100Adc from the battery system: when the Prosine is set to provide 200% of 200Ah it is essentially able to produce full charge power of 100A, and will support the extra DC load up to this 100A max while in float mode without significantly affecting the battery,
By contrast if you select a much lower Max Current %C setting such as 10%C(200Ah) or 20 Amps, the charger would only provide up to 20A total to support potentially a 30-100A load, therefore the difference must be provided by discharging the battery (unnecessarily).

If the charger is overwhelmed with a large parallel DC load during float and the load demands enough current from the battery to drop the system voltage down to the default 12.5V Threshold voltage/time out limit (defaults), the limits are intended to prevent excessively frequent repeat triggers into a new charge cycle, due to minor D.O.D which could result in excessive time spent at the higher absorption voltage for a battery still at 99.9% SOC. If you set the threshold voltage higher it will retrigger into a new charge/absorption cycle sooner. These set points and decisions are best described as a grey area of compromises rather than black and white,

• Max Timeout
This is the maximum amount of time that the charger will be in Float stage.

• Threshold Voltage and Timeout
When the battery voltage goes below this value for the Threshold Timeout
specified, the unit goes into Bulk charging mode.
Hope above helps .

Thank you
Best Regards,

Charging deep cycle batteries under load

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