Rectifier for golf cart type charger. Anyone worked on one?

[viper]

I am sort of "rolling my own" charger so I have flexibility in charge voltage.  I am using a buckboost transformer, 120pri, 32V sec, but that is RMS AC, when kicked to DC, I am right where I want to be for a 36V charge range. 

However, I need to rectify the AC after the transformer.  I was looking at golf type diode packs but they don't really look like full wave?  Anyone have knowledge or another cheap idea to rectify the power?  Sort of looking for more of a "module" than just axial lead diodes.  I will be moving about 40A on the 36V side. 

[CaptDon]

Buck / Boost  I am hoping your output winding is isolated from the A.C. Line!! Is your output 32VAC all the way across or is it a center tapped secondary?

[duak]

A friend of mine owns a cabinet shop and uses a walk behind forklift for small lift jobs.  The charger was thrown in and was too small for the size of the battery (24 V/500 AH?)  If memory serves it was a Lester unit that operated from a 120 V/15 A circuit much like this: https://assets-global.website-files.com/62b1d1ced3ce357ed11bccde/62b1d1ced3ce3512eb1bcec6_32698_E.pdf

It could and did grind out 25 amps for hours until the transformer finally failed.  I took it apart expecting a rectifier to have died but they were fine.  The rectifiers were stud mount style, with mica insulators on something like 1 x 1 x 3/16 x 4" Al 'L' extrusions bolted to the case.  I don't know the rectifiers' numbers or what they were rated for; at least 20 amps and 100 V?  Note, there were two rectifier assemblies with two rectifiers on each connected in parallel.  I believe the charger was for 48 V initially and modified for 24 V by connecting the secondaries in parallel.

An automotive alternator diode bridge assembly might be a good way to get a rectifier.  I can't recommend a particular style or manufacturer.  I can't guarantee the PIV ratings of the diodes, but they should be OK.  It will need a good heat sink or forced air cooling, depending on the design.

It might be better to just get a 50 amp bridge and be done with it, if such things are made,  A 1/4" tab is maybe a bit iffy for this current.

International Rectifier (and others) used to make rectifiers in modules that could be screwed to a plate or heat sink.  Connections were made with screw/ring terminals.

Big batteries have low internal resistances so you may have high peak currents when the battery is less than fully charged.  This could cause the transformer and rectifiers to run way hotter than expected.  The transformer in the Lester charger was ferro-resonant and acted as a current limiter as well as a voltage regulator.  Lester also had fuses in the secondary circuit just in case.  If a rectifier fails shorted, the battery will discharge into the secondary of the transformer and probably destroy it and the wiring.

Don't know if this helps, good luck!

[viper]

Buck / Boost  I am hoping your output winding is isolated from the A.C. Line!! Is your output 32VAC all the way across or is it a center tapped secondary?

If not familiar, you can look them up but yes, they are called "buck boost" when used for that, but when run in isolation config, they are isolated. 

[viper]

A friend of mine owns a cabinet shop and uses a walk behind forklift for small lift jobs.  The charger was thrown in and was too small for the size of the battery (24 V/500 AH?)  If memory serves it was a Lester unit that operated from a 120 V/15 A circuit much like this: https://assets-global.website-files.com/62b1d1ced3ce357ed11bccde/62b1d1ced3ce3512eb1bcec6_32698_E.pdf

It could and did grind out 25 amps for hours until the transformer finally failed.  I took it apart expecting a rectifier to have died but they were fine.  The rectifiers were stud mount style, with mica insulators on something like 1 x 1 x 3/16 x 4" Al 'L' extrusions bolted to the case.  I don't know the rectifiers' numbers or what they were rated for; at least 20 amps and 100 V?  Note, there were two rectifier assemblies with two rectifiers on each connected in parallel.  I believe the charger was for 48 V initially and modified for 24 V by connecting the secondaries in parallel.

An automotive alternator diode bridge assembly might be a good way to get a rectifier.  I can't recommend a particular style or manufacturer.  I can't guarantee the PIV ratings of the diodes, but they should be OK.  It will need a good heat sink or forced air cooling, depending on the design.

It might be better to just get a 50 amp bridge and be done with it, if such things are made,  A 1/4" tab is maybe a bit iffy for this current.

International Rectifier (and others) used to make rectifiers in modules that could be screwed to a plate or heat sink.  Connections were made with screw/ring terminals.

Big batteries have low internal resistances so you may have high peak currents when the battery is less than fully charged.  This could cause the transformer and rectifiers to run way hotter than expected.  The transformer in the Lester charger was ferro-resonant and acted as a current limiter as well as a voltage regulator.  Lester also had fuses in the secondary circuit just in case.  If a rectifier fails shorted, the battery will discharge into the secondary of the transformer and probably destroy it and the wiring.

Don't know if this helps, good luck!

Hmm, interesting.  I don't know anything about the old chargers like Lester.  Current limiting, regulation, all news to me.  Would like to learn more if you have any data.  Beyond that I don't think alternator packs can work because I haven't seen one for 36V.  It's an odd voltage.  I also know pretty much no charger OEM does any thermal comp or really anything advanced for bigger chargers.  Shocking but probably so they can sell more batteries. 

I do know these old but unused AGM batts are no longer "low resistance".  Pathetic really but we have them and will put them to work, even if only 50% cap.  Testing the batts, I will say they can only take about .1C without over volting them.  So 200ah batt is 20A charging.  I can push to 2.45V per cell before I just have to voltage limit the charge event.  Massive current is not going to happen. 

[inse]

Usually there is a note on AGM batteries about charge terminal voltage like xx.xV standby use and yy.yV cycle use.
The standby voltage can be left connected forever but will charge the battery very slow, the cycle charge voltage will charge faster but needs to be disconnected when full.
If you want both, fast charge and carefree float charge, you need something more complicated - see attached graph.
This method is the most beneficial one for battery lifetime.

[duak]

Lester has been around since the early 60s and the ferroresonant transformer since 1938 (Joseph Sola invented it).  Here's a blurb on the salient characteristics: https://voltage-disturbance.com/power-engineering/ferroresonant-transformer/  They may not be efficient, but like an anvil, can be effective and durable.  I looked inside a properly sized charger for an electric forklift and all the basic parts were the same, but larger.  It had some electronics to handle limit voltage and perform a cell balance.  Could have got it for free before it was loaded on a truck to get scrapped.

Digikey seems to have some bigger diode modules that could work, but they're expensive.  24 VDC alternators are available for trucks.  Generally, rectifier diodes have breakdown voltages above 100 V due to process improvements and the requirement to handle a load dump where the alternator is accidently disconnected from the battery. 

I mentioned fusing the secondary circuit because a diode could short and without current limiting, even an old battery can stuff enough current back into the charger to cause a hazard.

[viper]

I think what I am trying to determine is the actual circuit of these golf chargers to determine if those diode packs can work.  I can find them cheap enough, but appears to be only 2 diodes, but I can't imagine these are half wave bridges.  There is another component in the middle so just an unusual circuit for me. 

I'm not to concerned with inrush as I will have control of the primary voltage so I would roll up to my charge voltage. 

[calzap]

I think what I am trying to determine is the actual circuit of these golf chargers to determine if those diode packs can work.  I can find them cheap enough, but appears to be only 2 diodes, but I can't imagine these are half wave bridges.  There is another component in the middle so just an unusual circuit for me. 

I'm not to concerned with inrush as I will have control of the primary voltage so I would roll up to my charge voltage.

Circuit diagram of 24 V Lester charger is below along with a pic of the diodes.

Mike

[viper]

Wow, thank you!  I am still puzzled one how those rectifiers work, but that almost looks like an SCR pack?  Or I guess I am trying to understand the "sense" part of the circuit.  With only two diodes, that is half wave, which seems odd for all the power that is needed.  I would be expecting 4 diodes, so maybe I am missing something? 

[calzap]

The sense wires have two functions.  One is to protect the charger against a short circuit or battery connected in reverse on the output.  There has to be some voltage of correct polarity, somewhere about 1/2 the rated output voltage in my experience, on the outputs before a charging cycle will start.  A consequence is the charger can't charge a dead or almost dead battery.  In addition, the charger will periodically check the external voltage on the outputs to determine when to quit charging.  It also has an internal clock that will stop the charge cycle after a fixed time no matter what the voltage.

Mike

[PCB.Wiz]

Wow, thank you!  I am still puzzled one how those rectifiers work, but that almost looks like an SCR pack?  Or I guess I am trying to understand the "sense" part of the circuit.  With only two diodes, that is half wave, which seems odd for all the power that is needed.  I would be expecting 4 diodes, so maybe I am missing something?

The circuit was designed way, way, back when semiconductors cost more than copper. 
It ticks the KISS box, and looks to use widely available automotive alternator rectifier cells.

A bridge would mean twice the diode-power loss, and they are paying for a large multi winding transformer anyway, so a little more copper there does not bother them.