Trouble with charging a large 12v battery

[hey-hey-my-my]

Hello!

I recently upgraded the 110ah battery in my camper van for a 260ah AGM one.
My solar panels (200w via MPPT, soon to be doubled) struggle to keep up with my consumption, so once a week I hook up the battery to mains using CTEK's 5A charger, which supports AGM batteries but is not recommended for batteries larger than 110ah.

Why is this the case? I can indeed see that it goes into float mode way before reaching 100% - at which point I disconnect it, wait a bit, and reconnect. It goes back to bulk charging and the charge level ends up slightly higher than before, but never reaches 100%. (is doing this bad for the battery? The Ctek charger is supposed to be "smart" and not over charge)

The solar panels are able to get the battery up to 100% on a sunny day, even though I never get more than 9a/h out of them. This charge rate is still way less than the recommended minimum 10% of battery capacity (that would be 26A minimum in my case). How come 9A MPPT succeeds but 5A Ctek doesn't? Is there a charging threshold of sorts between 5 and 9 amps?

TL;DR Is using an underpowered mains charger (5A for a 260ah 12V battery) bad for the battery ; ineffective ; or just slow? I've searched around a lot and I can only see "not recommended" but no one seems to explains why.

Thanks in advance!

 

[Zerax]

Have you checked the setup within the MPPT solar controller and the CTEK charger ? AGM voltages will be different to normal wet cell voltages. And where are you getting your "%age charged" figures from ? These are all derived values... you need to look carefully at your voltages until you've built up confidence that the %age charged numbers are correct.
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If you check the spec of the new AGM battery... it'll tell you what the voltages are for bulk charging and float etc.

 

[dmxtothemax]

Typical charging currents are 10% of the amp hour capacity,
so 260ah should be charged at 26A for 12 to 14 hours
So even 9 amps is not ideal.

 

[hey-hey-my-my]

Have you checked the setup within the MPPT solar controller and the CTEK charger ? AGM voltages will be different to normal wet cell voltages. And where are you getting your "%age charged" figures from ? These are all derived values... you need to look carefully at your voltages until you've built up confidence that the %age charged numbers are correct.
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If you check the spec of the new AGM battery... it'll tell you what the voltages are for bulk charging and float etc.

Thanks for your reply. Yes, the chargers are both supplying the right voltages for AGM batteries. I'm fairly confident about my charge % observations. I disconnect the charger and all loads, wait an hour, then check voltage at the terminals. I never get more than 12.6V on the new AGM battery (it should be 12.8V).

My question is: why is a low amp charger (less than 10% of battery capacity) not recommended for a large battery?
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Typical charging currents are 10% of the amp hour capacity,
so 260ah should be charged at 26A for 12 to 14 hours
So even 9 amps is not ideal.

Thanks for your reply. Why is 9 amps not ideal? Is it harmful/useless to use my 5A charger on this battery, or does it just take a lot of time to charge?

Suppose I discharge my 260ah battery only by 10%. That uses up 26ah and leaves 234ah in it. My 5A charger should get it to 100% in under ten hours, right? (assuming a less than 5A charge rate due to the charger going into float mode).

 

[Zerax]

As long as the charger is capable of supplying the right voltages AND it is not set up to time out (some have max times built in) AND it's rated for continuous service... the charger should eventually charge the battery... assuming that there is no demand on the battery of course. But yes, it would be better to have a charger that can supply a larger current... to give faster recharge times

 

[hey-hey-my-my]

As long as the charger is capable of supplying the right voltages AND it is not set up to time out (some have max times built in) AND it's rated for continuous service... the charger should eventually charge the battery... assuming that there is no demand on the battery of course. But yes, it would be better to have a charger that can supply a larger current... to give faster recharge times

Good news then! I was worried that a low amperage would not charge the battery effectively, due to not shaking up the cells sufficiently or some other chemical fact that's beyond my understanding.

So I guess that explains why I have to disconnect my charger and re-start the process periodically : the charger times out and hops on to the next stage, regardless of the battery's charge level. I find it odd that the Ctek charger doesn't detect this and pretends everything is fine. A last question then: is it harmful for the charging voltage to go up to absorption/float levels before it's supposed to? I ask because due to the timeouts, the charger is assuming that it's in the final stages of charging when in reality it isn't; it's still somewhere around 75%. Then I start the process again and it goes up to 80%, and so on.

 

[Zerax]

Float charge voltage is lower than absorption voltage... so that's not harmful as such. As long as you're not discharging too deeply, you'll be fine. On my Victron MPPT charge controllers... they will bulk charge for as long as needed to get to the absorption voltage, then hold at that voltage for a period of time (max 2 hrs for me), then drop down to a float voltage. The current during the absorption stage should slowly drop off.

For AGMs... from memory, they like being charged slowly and hate overcharging...so you must read the specs and setups !

 

[marconi]

I have used a £800- £1000 battery bank of large 12V Ah (100, 200) Victron AGm batteries for about 10 off grid PV installations so far. AGM do require their own charging profile to maximise life and make the best use of in my case solar energy. It is important not to overheat the battery during charging so think Isquared R when considering the charge current. The chargers I have used monitor the voltage of the battery and the current and time elapsed to decide when to move to the next stage of charging:

The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity (25% of battery capacity in amp hours), and not raise a wet battery over 125° F, or an AGM or GEL (valve regulated) battery over 100° F.

The ABSORPTION stage (the remaining 20%, approximately) has the charger holding the voltage at the charger's absorption voltage (between 14.1 VDC and 14.8 VDC, depending on charger set points) and decreasing the current until the battery is fully charged.

The FLOAT stage is where the charge voltage is reduced to between 13.0 VDC and 13.8 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely.

So the appropriate charger is necessary and have the facility to be tailored to the battery it is charging.

This is one of a few references I used in my research on how to care for these batteries by careful charging and the bold text above is lifted from it:

Battery Charging Tutorial | ChargingChargers.com - http://www.chargingchargers.com/tutorials/charging.html

Important PS: MAke sure you have close to the battery a fuse/circuit breaker to rupture in the event of short circuit and that the battery terminals are shielded with plastic covers.

 

[hey-hey-my-my]

Float charge voltage is lower than absorption voltage... so that's not harmful as such. As long as you're not discharging too deeply, you'll be fine. On my Victron MPPT charge controllers... they will bulk charge for as long as needed to get to the absorption voltage, then hold at that voltage for a period of time (max 2 hrs for me), then drop down to a float voltage. The current during the absorption stage should slowly drop off.

For AGMs... from memory, they like being charged slowly and hate overcharging...so you must read the specs and setups !

That's great, thanks for your time Zerax!
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I have used a £800- £1000 battery bank of large 12V Ah (100, 200) Victron AGm batteries for about 10 off grid PV installations so far. AGM do require their own charging profile to maximise life and make the best use of in my case solar energy. It is important not to overheat the battery during charging so think Isquared R when considering the charge current. The chargers I have used monitor the voltage of the battery and the current and time elapsed to decide when to move to the next stage of charging:

The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity (25% of battery capacity in amp hours), and not raise a wet battery over 125° F, or an AGM or GEL (valve regulated) battery over 100° F.

The ABSORPTION stage (the remaining 20%, approximately) has the charger holding the voltage at the charger's absorption voltage (between 14.1 VDC and 14.8 VDC, depending on charger set points) and decreasing the current until the battery is fully charged.

The FLOAT stage is where the charge voltage is reduced to between 13.0 VDC and 13.8 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely.

So the appropriate charger is necessary and have the facility to be tailored to the battery it is charging.

This is one of a few references I used in my research on how to care for these batteries by careful charging and the bold text above is lifted from it:

Battery Charging Tutorial | ChargingChargers.com - http://www.chargingchargers.com/tutorials/charging.html

Important PS: MAke sure you have close to the battery a fuse/circuit breaker to rupture in the event of short circuit and that the battery terminals are shielded with plastic covers.

Thank you for the comprehensive response & tutorial link, Guglielmo. Makes sense. All positive wires leaving the battery terminal are fused appropriately in my setup, including the one going to the MPPT charger. Battery terminals are shielded at all times except when I hook up the mains charger (crocodile clips). This charger is fused on the AC side.

I'll look for a larger charger. If you have any recommendations as to what models are affordable (Ctek's fancy 25A model is beyond my budget) I'll gladly take them.

Cheers

 

[marconi]

Is this one within your budget range?

12V solar panels charging kits for caravans, motorhomes, boats, yachts, marine - https://www.photonicuniverse.com/en/catalog/full/383-20A-12V-Intelligent-automatic-multi-stage-mains-battery-charger.html

My B-I-L and I use PU to supply our off grid PV equipment and mains chargers. Perhaps give them a call? When we have phoned they have sometimes recommended or sourced items not on their shelves.

I cannot otherwise recommend a stand-alone charger 'cos we use inverter-chargers or hybrid solar inverters (which include a charger as well as PV controller) which are much better value-for money and compact.

I recommend you buy as we have done some insulated spanners, arc flash protection PPE. Folk tend to be naturally careful with dangerous voltages but these gel batteries can cause extremely dangerous currents of 1000s of Amps to flow if they are shorted which causes very, very, very rapid heating and plasma 'explosions' - in an instant.