Requirements for a DIY trickle charger?

Step-Hen

Observer
I have a 12V bank of Trojan T105s that only gets used sporadically. I'd like to use the Victron charge controller that currently sits on it with a more portable AGM battery, and put the Trojan bank on a trickle charger when I'm not using it. According to Trojan, maximum self-discharge for an unloaded T105 is 15% per month, although my Victron shows that it is delivering 0.3A once the float current levels off. Either way, it's not much.

If I understand correctly, if I put a 3A voltage-regulated power supply set at the recommended float voltage on the fully charged Trojan bank, the battery will simply pull current equal to the self-discharge rate...is that correct? I see a lot of marketing hype about expensive "intelligent" trickle chargers that prevent overcharging, but it that really necessary? The DIY trickle charger is $7 worth of parts, so is vastly preferred if it will work.
 

comptiger5000

Adventurer
If you're only planning to use it to keep the battery bank on a float charge and topped off while not in use, I see no reason that wouldn't work. Especially if you don't need to deal with large temperature swings where temperature compensation would be desired. Just make sure the bank is fully charged before putting it on the float charger.

Most of the reason for fancier chargers is that they can handle getting the battery up to full charge at a higher voltage and then drop to a float charge all in one unit.
 

dwh

Tail-End Charlie
if I put a 3A voltage-regulated power supply set at the recommended float voltage on the fully charged Trojan bank, the battery will simply pull current equal to the self-discharge rate...is that correct?

The amperage potential of the source is irrelevant. 3a or 300a, doesn't matter.

All that matters is that it's large enough to compensate for the self-discharge rate, and that the voltage regulation is set to the proper voltage.

But, for long term storage, the batteries will stay healthier if pumped up a bit on a regular basis, hence the charge profiles of an Iota with an IQ/4 module, or a Progessive Dynamics...both of which have timers to automatically cycle through the bulk/absorb stages after X hours at float.
 

Step-Hen

Observer
The amperage potential of the source is irrelevant. 3a or 300a, doesn't matter.

All that matters is that it's large enough to compensate for the self-discharge rate, and that the voltage regulation is set to the proper voltage.

But, for long term storage, the batteries will stay healthier if pumped up a bit on a regular basis, hence the charge profiles of an Iota with an IQ/4 module, or a Progessive Dynamics...both of which have timers to automatically cycle through the bulk/absorb stages after X hours at float.

Thanks for confirming what I thought was the case- sometimes it's hard to filter through the ambiguous & often misleading marketing hype around battery chargers.

From a battery chemistry standpoint, do you know why the periodic cycling through bulk/absorb is necessary? If the battery is initially topped off and the trickle charger supplies enough current to compensate for self-discharge, it seems like it wouldn't be necessary. But, as they say, reality trumps theory...
 

DaveInDenver

Middle Income Semi-Redneck
Thanks for confirming what I thought was the case- sometimes it's hard to filter through the ambiguous & often misleading marketing hype around battery chargers.

From a battery chemistry standpoint, do you know why the periodic cycling through bulk/absorb is necessary? If the battery is initially topped off and the trickle charger supplies enough current to compensate for self-discharge, it seems like it wouldn't be necessary. But, as they say, reality trumps theory...
Charge equalization is an understood process. If you start with a fully charged battery and get the trickle current right it probably isn't critical but as near as anyone can tell it doesn't seriously damage anything and does work. You are right, if you start with a new, equalized battery and treat it right all the cells should age similarly and reduce the need for future equalization.

The underlying cause is that a typical finishing (the 3rd charge stage) or float voltage across the battery (due to manufacturing variance) results in a string voltage where cells aren't really brought up to or kept at 100% SOC. The effect of this slight undercharged condition accumulates over time to further weaken those cells. If you apply an elevated voltage to a, say, ~99% charged battery it forces the partially charged cells to reach 100% and as long as you don't push the stronger cells too hard or too long they will tolerate the short overcharging period.

To reverse significant sulfation on the weaker cells you have to push the stronger ones pretty hard, which consumes some of their life. So a true equalization (FWIW, equalization, conditioning, de-sulfation, there's not really a universal definition) is not something you want to do often. The approach Iota takes with IQ4 is to apply the absorption voltage once every 7 days for 8 hours (I think) so that you hopefully never have to do a 15V+ equalization. This is not as harsh so they can do indefinitely and not significantly impact the life.

The main problem there is most people don't leave their batteries on a charger for weeks, which is what Iota intends. It looks at the OCV of the battery to decide which step to start, so if you put an Iota on a fully charged battery it will sit at float for a week before you get an equalization. That's really the main thing I don't like about my DLS-45, no way to force an equalization.

But if you're running an RV or have a daily driver where your truck can sit on shore power the IQ4 chargers are good. I have a group 27 AGM that runs my ham station, which has been on a DLS w/ IQ4 for 9 years now and it's still strong enough to run my Engel fridge for several days. So it's still got to be somewhere around 90% of capacity (based on running about 24 A-Hr for 3 days, it's a 92 A-hr battery). It just sits on the charger 24/7, which is sized well beyond what my radios will ever draw so the battery doesn't have to work much.
 
Last edited:

Step-Hen

Observer
The underlying cause is that a typical finishing (the 3rd charge stage) or float voltage across the battery (due to manufacturing variance) results in a string voltage where cells aren't really brought up to or kept at 100% SOC. The effect of this slight undercharged condition accumulates over time to further weaken those cells. If you apply an elevated voltage to a, say, ~99% charged battery it forces the partially charged cells to reach 100% and as long as you don't push the stronger cells too hard or too long they will tolerate the short overcharging period.

To reverse significant sulfation on the weaker cells you have to push the stronger ones pretty hard, which consumes some of their life. So a true equalization (FWIW, equalization, conditioning, de-sulfation, there's not really a universal definition) is not something you want to do often. The approach Iota takes with IQ4 is to apply the absorption voltage once every 7 days for 8 hours (I think) so that you hopefully never have to do a 15V+ equalization. This is not as harsh so they can do indefinitely and not significantly impact the life.

Thanks, good info! Makes sense.
 

Forum statistics

Threads
185,527
Messages
2,875,534
Members
224,922
Latest member
Randy Towles
Top