PWM Circuit for AGM Battery Charger
PWM Circuit for AGM Battery Charger
(OP)
Hi, For different reasons, I am designing a custom battery charger for an 12Vdc 100amps AGM battery. I use a PWM control circuit, but the driving Mosfet tend to heat a lot. The actual circuit simply switch the input DC source (solar panels) using a power mosfet directly in the battery trough diodes. I tough that adding a coil between the mosfet output and the battery may help to lower the heat dissipated but the mosfet. I need help figuring out how to compute the proper inductance value in relation with my switch frequency. So any help will be welcome. Maybe I am wrong, and the coil will not help at all. Your input will be welcome. Input DC voltage vary from 12 to 22Vdc approximately while the battery will peak around 15Vdc, I try to keep current constant using the PWM. The circuit is working, but heat a lot. Peak current may raise to 10amps when running in bulk charge.
Bye
Jacques
Bye
Jacques





RE: PWM Circuit for AGM Battery Charger
RE: PWM Circuit for AGM Battery Charger
Bye
Jacques
RE: PWM Circuit for AGM Battery Charger
array. If you have more than one panel put them in series.
RE: PWM Circuit for AGM Battery Charger
Bye
Jacques
RE: PWM Circuit for AGM Battery Charger
Bye
Jacques
RE: PWM Circuit for AGM Battery Charger
Just curious as to the manufacturers recommended charging rate for your battery.
Also a bit puzzled by your current sense resistors of 3 X 0.033 Ohm in parallel, but a listed voltage range of 0 - 3.3 volts. Is this your current design drawing?
RE: PWM Circuit for AGM Battery Charger
RE: PWM Circuit for AGM Battery Charger
While batteries seem like simple devices, the chemistry of AGMs especially considering the gas recombination process is quite complex and demands the resultant float voltage to be correct for the battery temperature.
If resultant float is off in one direction, positive electrode (plays a more significant role in the gas recombination than the negative and, there is a heating effect and delta voltage effect on terminal voltage from the gas recombination itself). If off the curve, positive electrode corrosion tends to occur; if off in the other direction, sulfation build up is likely.. This aspect was always question number 1 when approaching the battery manufacturer for a warranty replacement.
All the battery manufacturer's in my experience specify float versus temp for their AGM stationary batteries (measured on the positive terminal) to 2 significant digits and of course all modern commercial microprocessor controlled chargers have the selectable or customizable profiles in their design to match the battery manufacturers requirement.
One last reason for this with AGMS is the possibility of battery thermal runaway if the batteries are in an uncontrolled temperature environment where battery temps can get up to 95 deg F or beyond. AGMS as they get hotter start drawing more charging current for a given charger voltage and without remote battery temperature sensing by the charger, a thermal run away can happen..
I've seen AGM thermal runaway happen 3 times at commercial customer sites, in one case destroying about $25,000 dollars of commercial batteries where the customer refused to replace their aging non compensated chargers with newer technology. We ended up placing temperature indicating stickers www.wahltempplates.com on the battery systems on all our accounts and cautioned the customers about this, in that if the system were damaged, it would be at their expense.
RE: PWM Circuit for AGM Battery Charger
RE: PWM Circuit for AGM Battery Charger
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RE: PWM Circuit for AGM Battery Charger
RE: PWM Circuit for AGM Battery Charger
The recommended bulk charge rate is around 10% of the battery capacity of 100amps so around 10amps. I design the circuit to handle 15 amps, but I limit the current to not more than 10amp via the PWM.
The 0-3.3V in relation with the sense resistor come from an ADM4073T.
I did switch R8 from 100K to 1K but forget to correct the schematics.
My PWM frequency is 1Khz.
I do not use UNO, the PWM is drive by an Atmel 89C5130 MCU.
I did implement a temperature sensor to control AGM charge.
Battery temperature will stay low, they are surrounded by water not going over 10c.
Correct me if I am wrong, but Q1 (IRF4905) has a 20mohm Rds, so at 15amp, it should drop around 0.3watt, not 105. It's not a linear regulator that in fact have to drop 7 volts.
The parts that heat a lot are the sense resistors, Q1 & D20 that are on the same heatsink. I can easily raise the resistors power, but it's a bit more complicated to dissipate the heat produce by Q1 and D20.
Q1 should only produce 0.3Watts
D20 have been replace by a MBR2515L with 450mv drop voltage so it should dissipate around 7 watts at max power.
I wish I can find a diode with a lower drop voltage for D20, this should reduce the heat.
Sorry if the schematic was not up to date with all the modifications I did during the test period, and I did not post the full schematics, the MCU is not relevant to the heat problem except for the PWM frequency.
I will correct the software to remove short pulse as suggested.
Bye
Jacques
RE: PWM Circuit for AGM Battery Charger
If the input is 22V and the output is 15V, then yes, you're dropping 7V. If your calculation is correct, then there should be no reason for the transistor to get hot. Since it is getting hot, then the calculation may not what you think it is.
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RE: PWM Circuit for AGM Battery Charger
IR, solar panels are current sources. In simple PWM they just short them to the battery
RE: PWM Circuit for AGM Battery Charger
Darrell Hambley P.E.
SENTEK Engineering, LLC
RE: PWM Circuit for AGM Battery Charger
But, if you expect 15V output with 12-22V input then maybe take a look at buck-boost type circuits.
RE: PWM Circuit for AGM Battery Charger
OK, but, nevertheless, they are not instantaneously compliant; there's always a response time that's dependent on series resistance and junction capacitance, particularly on a large panel. For however many nanoseconds or microseconds it takes to actually discharge the junction capacitance, the MOSFET is dumping a truckload of power.
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RE: PWM Circuit for AGM Battery Charger
1. High peak currents (EMI issues, increased losses)
2. High ripple currents in the battery (increased losses, heating, etc)
3. Lack of safe charging and control
4. Cannot charge the battery when your input voltage is low
A simple buck converter will solve 1-3 but to deal with 4 you will need a more complicated topology. Further information about power levels, isolation requirements, etc is needed as well. To do this right is not a simple task.
RE: PWM Circuit for AGM Battery Charger
RE: PWM Circuit for AGM Battery Charger
Are you building this from scratch without referring to tried-and-true designs?
When Xantrex built their C40 solar charge control / voltage regulator system (in the 1980's) they went with IGBTs, not MOSFETs.
Everything you want your controller to do, is about 1/4 of the functions possible in the Xantrex device. You can still buy them for about 50 USD.
If you want MPPT, then get a Tristar for a little more money, and it has 2x the functions as the Xantrex C40.
STF
RE: PWM Circuit for AGM Battery Charger
First thanks for all the comments, every thought is useful.
Reading your ideas, searching the internet, thinking about the current design make me realised that my actual concept may not be the best way to go for my application.
The thread starts while I was thinking to add a coil to the design in the hope to raise effectiveness. Heat is annoying and a lost of energy in this case. My design requires the best energy use possible.
My goal is to have the possibility of turning the charger to charge or float mode and use the maximum energy possible from the solar panels, while not producing unwanted heat in the process.
From all the thoughts, what is appealing to me is the idea of using a Buck-Boost switcher. I found one example using a LM5118 that please me. This concept should be a lot more efficient and should not produce a lot of wasted heat. It will change the battery even if the solar voltage is under 12Votls, so using every watts available even if it’s not much.
I add the schematic to the thread, I found it on internet, I do not know who done it, but it is now public.
Please comments on the idea, I may try to build this for experimentation.
Bye
Jacques
RE: PWM Circuit for AGM Battery Charger
Bye
Jacques
RE: PWM Circuit for AGM Battery Charger