Possible mod to old school battery charger?  Need real voltage regulation - Page 1
Forum section: electronics / projects

author: viper
author_rank: Regular Contributor
timestamp: February 06, 2022, 07:31:25 am
content: So I am doing more and more work with batteries of various types. FLA, AGM, Li, etc. But for right now I am focused on AGM as I need to recharge a bunch of AGM batteries and trying to use an old school charger. These are all designed with just a transformer, diode packs, and an amp meter. In short, they are virtually guaranteed to over volt a battery, every single time! As a battery charges, the amperage will decrease so the voltage will increase. I have seen over 16V. AGM will not tolerate this!Now you go take one look at ebay for "smart chargers" and we have a bunch of over priced junk. No clue how they operate, what functions they have, etc. I am perfectly fine to decide my own voltage parameters, and would just love an absorption timer, then roll to float, but I could do without for now.What I am doing right now is having to put the charger on a lower setting just to throttle the voltage. This is a 100A charger and that still is not really enough. However, I am getting only about 30A from it because I have to watch the voltage.So unless you guys have other ideas, I was considering using this old turd, and try to sneak in some voltage regulation? It would need to be adjustable as different chemistries and temps have different requirements. I guess I look at this as the old charger has the transformer and diode packs. Not actually sure how clean the power is, but probably not clean enough anyway.Or maybe find a better charger? IDK! I DO know that solar MPPT charge controllers have ALL of this, can run bigger amps, and not all that expensive, but they do not have and AC converter front end in them. But they can certainly take about any DC voltage in and regulate it to charge batteries. Just seems sort of silly.

author: inse
author_rank: Super Contributor
timestamp: February 06, 2022, 08:31:22 am
content: Your charger with all its limitations has specifically been designed for the job 'charge a beefy lead battery' in combination with the operating instruction: disconnect when battery is sufficiently charged.Adding a regulator for 100A on the secondary side is definitely a challenge, just think of the power dissipation and short circuit protection.Check the feasability of using the charger as supply for the charge controller you mentioned, but you will have to add 'some' capacity to the rectifier.Depending on the power supply quality, calculate 1000 to 2000uF per Ampere :-o

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 06, 2022, 12:58:27 pm
content: Add a SSR on the input to turn it off for a few seconds when the voltage is high enough.

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2022, 01:20:05 pm
content: Also you may need a high power low-ohm 'ballast' resistor in series with the charger output to limit the max. current to less than 20% C (5 hour rate) which is typically the highest permissible charging current for AGM batteries. They are *NOT* traditional flooded cell batteries so don't respond well to being 'boiled' by high current charging. Finding the right resistor value can be difficult without considerable experimentation checking charger output voltage droop with respect to load current, and battery impedance with respect to state of charge. If deep discharged, the charging current can creep up as the internal resistance can drop faster than the counter-EMF from the open circuit cell voltage increases.

author: SeanB
author_rank: Super Contributor
timestamp: February 06, 2022, 01:36:19 pm
content: There is a circuit that uses a beefy SCR to limit charge current as the battery voltage rises, with the output voltage being compared by a zener diode to a sample, and this then turning on a transistor as the battery voltage rises, and this in turn starving the SCR of gate current, so it tapers off as the voltage rises, finally stopping charge at your selected full battery voltage. Think this is an old ETI or Elektor design, around for a long time.