current limiting 12v charging circuit  - Page 1
Forum section: electronics / projects

author: tatersalad
author_rank: Contributor
timestamp: January 29, 2019, 03:21:32 am
content: Im stumped and so are my buddies at work. Im trying to charge 3600ah worth of batteries on a motor home. I have it set up with dual 180 amp alternators and a few inverters to run the ac units. I would like to limit how much amperage the batteries can draw so if they are low they wont over load the alternators and burn up the windings. any ideas? thoughts? Basically a dc to dc battery charger rated for 200 amps. Ive found a 40 amp dc to dc but its 400 and it would take a week to charge it back up. I have a generator but id rather not use it. thanks in advance for any and all advice.

author: mvs
author_rank: Frequent Contributor
timestamp: January 29, 2019, 09:19:07 am
content: Quote from: tatersalad on January 29, 2019, 03:21:32 amAlternator should be controlled through its field winding. There is no point in creating external dc-dc charger, because of the complexity, costs and worser efficiency.

author: soldar
author_rank: Super Contributor
timestamp: January 29, 2019, 07:54:31 pm
content: Quote from: tatersalad on January 29, 2019, 03:21:32 amYou are making this too complicated when it is quite simple. Forget about the batteries. You want to limit the alternator output to a safe limit. That is all. And there is a good chance the alternator already has this internally built in. You should find out what alternator you have and what kind of control/regulator it has.

author: tatersalad
author_rank: Contributor
timestamp: January 31, 2019, 09:11:49 pm
content: they are 22si alternators, the gentleman who built them says they are good for 250amps each but if they are run at a constant full load it will burn the regulator and diodes up he recommended 150 to 175 each and they will last for ever. I don't want to over complicate it but I also don't want 500 dollars of alternators ruined and a tow bill of 750. someone mentioned controlling charge voltage and that would also regulate the charge current so I might go that route. still open to ideas

author: soldar
author_rank: Super Contributor
timestamp: January 31, 2019, 09:17:06 pm
content: Quote from: mvs on January 29, 2019, 09:19:07 amThat is the simple and correct answer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2019, 10:26:14 pm
content: Exactly. There's a regulator module, often internal to the alternator. At worst, you could replace this with a custom module which regulates output current as well as voltage.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: January 31, 2019, 10:35:01 pm
content: Get a smart external alternator controller that can monitor the current via a shunt and also the alternator temperatures via a thermistor in contact with one of the stator coils. There are several companies that build such controllers for the marine market. Program the max load current and max temperature you want. Have your alternator builder break out the field brush connections (see controller manual for whether it needs one or both) and fit the thermistors. If possible fit a 'limp home' switch that disconnects the external field controller and reconnects the internal regulator in case the external controller fails.

author: floobydust
author_rank: Super Contributor
timestamp: January 31, 2019, 10:56:00 pm
content: 22SI is kinda old, newer alternators are brushless and have more sophisticated voltage regulators, can take higher temperatures (dual fan) etc.In reality the wiring, fusible-link, connections resistance limits possible charging current. Even 8 milliohms is enough.An easy way to lower output current is to lower shaft RPM. You'll also get less cooling at lower speeds. Just run it at lower speed.The only other way is to build a circuit that monitors alternator output current and spoofs the voltage-feedback up, so the alternator thinks it's at say 13.8V but it's only 12.8V this is a way to back off the output by fooling it.

author: soldar
author_rank: Super Contributor
timestamp: February 01, 2019, 08:13:56 am
content: Quote from: floobydust on January 31, 2019, 10:56:00 pmI assume you mean by fitting a larger diameter pulley. That seems to me like a bad idea that will not work well. If you look at the graph you can see the alternator would need to be kept at under 3000 RPM at all times which means most of the time it would not be putting out anything at all. Bad idea.Again, the field current is what controls the output current. Have a look at the regulator and there is a good chance you can easily mod it to do whatever you want it to do.In my boat I did the reverse. I wanted the alternator to output full amps all the time rather than start cutting back as voltage went up. The regulator is a simple circuit with a few transistors and I found I could add a switch which would have the field at full amps. If I wanted to limit output I could just increase the value of a resistor and that would limit the current through the field winding and thus the output. The OP is making this complicated and wrong because limiting what goes into the batteries without regard to what is going elsewhere is obviously useless.

author: floobydust
author_rank: Super Contributor
timestamp: February 01, 2019, 10:34:27 am
content: I was thinking lower engine speed could be used. Unless OP has to be on the road driving, but 3600Ah of batteries must be huge.Oversizing the alternator pulley is something street/strip racers do. At idle the alternator is cutoff. It's kind of silly as input torque is set by the voltage regulator, unlike a water pump which goes up with RPM.It seems alternator's output is limited by the belts and rectifier diode/winding heat.A battery isolator might be used to switch in and charge part of the pack, not all at once, between the two alternators. The 22SI is only 55% efficient so two at 150A will be a heavy load for an engine and belt arrangement. It should squeal loudly...Otherwise, adding line resistance or a custom voltage regulator would be needed. That's all I can think of.

author: tatersalad
author_rank: Contributor
timestamp: February 04, 2019, 11:03:13 pm
content: I wasn't sure what the best approach was, hints why I reached out here and asked. im by far no super hero when it comes to stuff but I can normally work through it and make something work. I thought about regulating charge voltage to regulate the amperage but wasn't sure if I really wanted to start another project and risk damaging a brand new alternator. I will be looking into the premade regulators. thanks for all of your feed back on this.For anyone who is wondering... im converting a school bus into an rv, I was given 36 12v batteries that are 2 years old and have had a very nice life in a battery back up unit at my work. they will be mounted underneath the bus in a battery rack to power stuff while boondocking. brushless alternators with higher amperage are on the list but other stuff needs attention first as the conversion isn't complete yet.

author: floobydust
author_rank: Super Contributor
timestamp: February 05, 2019, 08:06:01 pm
content: The 22SI is the old Motorola linear voltage regulator, I think it was discrete transistors then the MC3325 IC inside, thenCS3341.I looked at more modern alternator regulators and none of them look at output current. Some use nearby rectifier temperature to see if things are too hot, or knowing RPM and field current they model the output current. The industry has gone to LIN serial bus so the ECU can communicate to the alternator.They all use low frequency 50-400Hz PWM to excite the field winding.OP could use a hall-effect DC current transducer and a circuit or MCU to control the alternator or make a custom regulator, but these are a lot of work.I think I would just add some small resistance to limit current.NXP TC80310Alternator regulator with LIN (but die only).MC33099

author: tatersalad
author_rank: Contributor
timestamp: February 07, 2019, 09:17:33 pm
content: perfect thank you for the information ill post back with my findings when the rain dries up a bit and im able to work on it