Need help with best practices for charging up and monitoring my deep cycle battery

[DaveM]

Been having charging issues with my Interstate SRM-24 deep cycle batt that I thought meant the batt was bad (battery seems to discharge quickly even when not in use). Had Interstate load test it overnight and it seems ok. Now I'm thinking the issue is a combination of my charging and monitoring equipment, and my knowledge. So looking for some very basic help and info on how to properly charge, maintain, and check my deep cycle battery...

Battery - Interstate SRM-24, flooded, 81Ah, 1 year old.

Typically I charge the battery at home on a NOCO Genius charger to 100%, then let it sit in the trailer hooked up to the fuse box but disconnected via marine disconnect switch. I check it periodically and plug it back in to charge anytime the battery shows below 85% on the little batt box meter (this seems to be happening faster then it should when not in use).

Questions:
At home charging - I use a NOCO Genius 3500 (3.5 amp) charger.
Is 3.5 amps enough for charging this battery? I have read 2-3 amps is ok (just slow), have also read it should be 10 amps to get a true full charge on deep cycle batts. Otherwise I'll get a surface charge that looks like its 100% but fades quickly.
The Genius 3500 supposedly has a "maintenance" mode and can be hooked up continuously. Should I be doing this w my batt or disconnecting once fully charged as I have been?

Battery monitoring - I usually rely on the percent charged feedback from a small cheap gauge installed on the batt box when out camping. It seems to be unreliable (shocker). Is there a better way to monitor state of charge when out camping and charging off the panels, beside bringing my multimeter with me?
When I do use my multimeter my understanding is 12.7 is 100%, 12.5 is about 80%, and 12.2 is about 50%. I should not let the battery discharge more than about 50%? But I seem to hit 60% very quickly in the field (at least based on the cheap gauge).

 

[billiebob]

Following cuz this is one of those things that are alien to me.
Tell me what to do I'll do it religiously.
Explain how it works.....

Sorry thats like explaining marriage and sex and space travel to me.

 

[workerdrone]

3.5 amps is definitely enough current to charge, it will just be slower than more amperage.

12.7v resting voltage is considered 100% charged but I believe my charger is set to achieve 14.6v when charging - so make sure that your charger is getting up well past 12.7 in the bulk charge period.

I think lead acid will last longest if it spends as much time as possible near 100% state of charge, so no worries about overcharging like your phone or your tesla lol

 

[DiploStrat]

This is a bit dated, but is still good for lead acid. https://cookfb.files.wordpress.com/2016/09/battery-charge-slides.pdf

-- Recharge fully, any time you can, every time you can.

-- For longest battery life, try to limit discharge to about 50%. ALWAYS recharge immediately after a deep discharge - don't sit partially discharged.

-- Voltmeter is interesting, true battery meter is essential. BUT, it is less important that you know the state of charge, and more important that your charger know the state of charge. Make things as automatic as possible.

 

[Stitebunny]

Do you know if they actually fully charged it and used a real load tester to simulate a load? As opposed to an electrical battery analyzer.
I’ve seen many batteries pass an analyzer teat but actually fail a real load test.
It sounds like you are doing it ok.
I would not trust a volt reading as a state of charge though. Ideally you want an actual battery monitor. It would do a much better job.

 

[DaveM]

Do you know if they actually fully charged it and used a real load tester to simulate a load? As opposed to an electrical battery analyzer.
I’ve seen many batteries pass an analyzer teat but actually fail a real load test.
It sounds like you are doing it ok.
I would not trust a volt reading as a state of charge though. Ideally you want an actual battery monitor. It would do a much better job.

They did some overnight test that I think just involves bringing it up to full charge then checking it after the surface charge dissipates. They also tested with hydrometer. Everything looked fine but I told him the issue is it drops quickly over a week or two from 12.7 to 12.5 (100%-80%). So now he's doing the actual load test to verify.

 

[DaveInDenver]

They did some overnight test that I think just involves bringing it up to full charge then checking it after the surface charge dissipates. They also tested with hydrometer. Everything looked fine but I told him the issue is it drops quickly over a week or two from 12.7 to 12.5 (100%-80%). So now he's doing the actual load test to verify.

Bringing a battery to full charge and waiting overnight (called resting, usually 24 hours, which as you say allows the surface charge to dissipate) is the accepted method to judge initial state of charge.

That it drops 0.2V in a week is excessive self discharge. You'd expect on the order of 1% per week at 25°C, so dropping to 12.5V would take more like a month or two for a battery left unconnected in storage.

 

[Ngneer]

Sub’d very interesting, Thanks to all who have posted, great info

 

[jonyjoe101]

My advice is get either a TR16 coulomb counter or tk15 coulomb counter (both cost about 30 dollars for 50a version) with this meter you program your battery amp hours and then it counts the amps in/out of battery. I used these on my lifepo4 but they also work on lead acid. With these you can monitor how much amps you are using, if you use 20 amps, then next day you know you need to put 20 amps back in, if your charging system isn't fully charging your battery you know you got a problem.

You mention that you charge your battery with solar when out on the road, the solar controllers are very unreliable for charging any batteries. Evey charge controller I used had too much voltage drop from controller to battery. For instance the controller would read 14.4 volts but the battery terminals would be around 13.8 volts (float voltage), the controller thinks the battery is fully charge and switches to float voltage (13.2 volts) and the battery hasn't even got a full charge. To compensate you would have to raise the bulk voltage on your controller to a higher voltage.
On my ecoworthy 20 amp mppt, I had to raise the bulk voltage to 15.5 volts to get 14.4 volts on the battery terminals. When you mentioned that the battery drains quickly after it was charge, to me that sounds like it wasn't fully charged. A coulomb meter would help you there since it measures the voltage directly from the terminals ( the shunt is placed very near the terminals ).

I mainly charge my batteries with solar and didn't realize I was undercharging my lead acid batteries until I switched to lithium. With lithium there is no such thing as surface charge, they are either charge or not charged and I wasn't getting them fully charged with my solar controller. That's when I discovered the voltage drop and realized why my lead acids never lasted long and where giving me terrible performance. With solar I was never charging them past 14 volts.

 

[TwinStick]

I am certainly no expert but have been maintaining lots of batteries over my 59 years. I have almost $1000 worth of battery chargers. Seriously. The ones I use the most and like the most, believe it or not, is the HF Viking 4 amp charger maintainer. It is the only one that charges my batteries to 13.6v and maintains it. My Odyssey 50 amp triple digital charger does not, Dewalt charger does not, wheeled Shumaker does not, Sears trickle chargers do not, etc., etc. .

I have 6 of the HF Viking 4 amp charger maintainers. They have a winter mode, so you can leave them hooked up indefinitely. I love em. I go out to garage periodically and check with my Klein multimeter. I have 4 gp 31 agm batteries, snowblower battery, truck battery.

Keeping them topped off is key to long life. 1 of my AGM batteries is a Sears Marine die hard platinum from 2006 & is still going strong. Hope you get your batteries up to snuff. Keep us posted please.

 

[DaveM]

My advice is get either a TR16 coulomb counter or tk15 coulomb counter (both cost about 30 dollars for 50a version) with this meter you program your battery amp hours and then it counts the amps in/out of battery. I used these on my lifepo4 but they also work on lead acid. With these you can monitor how much amps you are using, if you use 20 amps, then next day you know you need to put 20 amps back in, if your charging system isn't fully charging your battery you know you got a problem.

You mention that you charge your battery with solar when out on the road, the solar controllers are very unreliable for charging any batteries. Evey charge controller I used had too much voltage drop from controller to battery. For instance the controller would read 14.4 volts but the battery terminals would be around 13.8 volts (float voltage), the controller thinks the battery is fully charge and switches to float voltage (13.2 volts) and the battery hasn't even got a full charge. To compensate you would have to raise the bulk voltage on your controller to a higher voltage.
On my ecoworthy 20 amp mppt, I had to raise the bulk voltage to 15.5 volts to get 14.4 volts on the battery terminals. When you mentioned that the battery drains quickly after it was charge, to me that sounds like it wasn't fully charged. A coulomb meter would help you there since it measures the voltage directly from the terminals ( the shunt is placed very near the terminals ).

I mainly charge my batteries with solar and didn't realize I was undercharging my lead acid batteries until I switched to lithium. With lithium there is no such thing as surface charge, they are either charge or not charged and I wasn't getting them fully charged with my solar controller. That's when I discovered the voltage drop and realized why my lead acids never lasted long and where giving me terrible performance. With solar I was never charging them past 14 volts.



View attachment 701896

Just bought and installed the tr16 unit. I think this is going to help significantly in giving reliable info on the SOC for my batt. I also suspect that you're right about under charging from solar and my Genius 3.5 amp charger. Both seemed to indicate the batt was full when it was really only at maybe 90% or possibly less. Interstate was able to get it charged fully, up to 13.2 or so once the surface charge dissipated. It's holding steady at about that amount after 1 week. Before it would have dropped to 12.6 after a full charge on the Genius. I'm ok if the solar charger isn't fully charging, it's just for topping off and stretching the capacity on longer trips (4-5 days). But I need a home charger that I can trust to fully recharge once back.

 

[jonyjoe101]

With the TR16 it will definitely help you in maintaining your battery. If you see the solar isnt getting the battery terminals up to 14.4 volts, I would definitely consider raising the bulk voltage higher if you can depending on what controller you have. Same with your AC chargers, you need to see the battery terminals get up to 14.4 volts and stay there for a while(absorb).
If your controller doesnt have manual settings that might be something to get next. I have the makeskyblue 60a mppt (cost 130), it has manual bulk setting and it also allows calibration of the controller with the voltage on the battery terminals, that feature alone is worth the price. The calibration feature works extremely well, the controller and battery terminals are practically the same voltage.
I now use lithium for the house battery and fully charging the battery is not required, but with lead acid, you need to fully charge it back every day, or at least topped off to 14.4 volts.

 

[DaveM]

With the TR16 it will definitely help you in maintaining your battery. If you see the solar isnt getting the battery terminals up to 14.4 volts, I would definitely consider raising the bulk voltage higher if you can depending on what controller you have. Same with your AC chargers, you need to see the battery terminals get up to 14.4 volts and stay there for a while(absorb).
If your controller doesnt have manual settings that might be something to get next. I have the makeskyblue 60a mppt (cost 130), it has manual bulk setting and it also allows calibration of the controller with the voltage on the battery terminals, that feature alone is worth the price. The calibration feature works extremely well, the controller and battery terminals are practically the same voltage.
I now use lithium for the house battery and fully charging the battery is not required, but with lead acid, you need to fully charge it back every day, or at least topped off to 14.4 volts.

Thanks. I had been using a crappy cheap SC from Amazon but just upgraded to the SunSaver 6 (https://2n1s7w3qw84d2ysnx3ia2bct-wp.../wp-content/uploads/datasheet-sunsaver-en.pdf). Have not used it IRL yet but spec sheet says 14.4 for 12v flooded as charge output.