Lead acid protection circuit

Question

Hi everyone,

I'm always looking for alternatives for a DC supply, as we have a constant problem with mains supply failure.

Add to this that I am a Ham operator who like to keep things up and running, even on DC power if required.

For DC supply I have the normal AC to DC switch mode supplies but the problem is with a suitable protection for my (very expensive according to me) AGM 12VDC batteries.

If you fail to constantly monitor the AGM battery output voltage or with swapping batteries accidentally (and it do happens if you are in a hurry at night) connect it reverse polarity you have a problem.

I have read many articles on battery protection especially with RPP and current flow back prevention as well as a battery low voltage cut-off so I'm starting a project that is compact and cost efficient enough to add to all my SLA's and AGM (and also all flooded cells for that matter) 12V batteries.

So my circuit is more a combination of several other circuits combining an efficient RPP without to many heat losses as well as combining it with a ATTiny ADC to read the voltage to cut off the battery at a preset low voltage, nothing very extravagant or to large or difficult to populate on a single sided PCB and fit it on the battery output.

My requirements would be to feed a radio or other load with DC at a maximum of 30A @ 13.8VDC (as required by my radio when transmitting.

The circuit area I'm not very sure of is the cut-off feed from the ATTiny45 via the 2N7000 that will cut the ground from the main P-ch MOSFET as a high side switch with a maximum current of 55A, Maximum gate voltage of 20V and a RDS(on) of 20mOhm at Vgs of -10VDC.

The second problem may be an Inrush current needed by the load which I do not totally want to limit as in certain instances my radio may require but I have to protect the MOSFET.

The other thing maybe to place a uni directional TVS at the battery poles of the circuit e.g. 1N6377 to clamp high voltage peaks etc.

Please advise me on the following circuit: (its available on PDF format here https://drive.google.com/drive/folders/1jG2ML2XMi52CiVaMMP-QXK-w0ViSZoYC?usp=sharing ) as well as the IRF4905 datasheet

dougw · Answer

Interesting application. You could also use something like an LTC4352 to create an ideal diode which also has under voltage protection. An alternative is LM74700.

jw0752 · Answer

Hi Eugene, You may want to check and verify the polarity of the IRF4905 as in its current config the internal Schottky will conduct current battery to load. As far as the rest of the design I will defer to some of the other guys with more design experience than me. John

ldjohn · Answer

Hi there Eugene, one scenario is when you try to switch off the load, the internal diode will conduct inside the mosfet, thereby continuing to conduct despite your efforts to stop it.

1) consider placing back to back mosfets in your design to isolate the load, because of the diodes.

2) consider adding another circuit in to allow a PSU to be connected as well as managed.

3) in certain scenarios where you have equipment that fails ( radio, charger, PSU) it may be good to allow under and over voltage protection. Sometiems a charger gets stuck and takes the battery over 16V. In your scenario, simply add another channel to the A/D to read input voltage for PSU and another set of back to back mosfets to control the output.

4) during high transient cut over currents, it may be pertinent to investigate limiting of inrush current by adapting the resistive load between Pchan gate and transistor pulling the circuit to ground for activation. This will create a small ramp time. Once engaged, no more ramp needed so thats good.

5) make sure that you put protection on the inputs of the A/D converters. i usually place a 5v1 Zener Diode (assuming 5v logic) between the A/D input and ground so that it limits maximum input to the A/D. This is also a great trick as if things go really wrong, the A/D pin is protected from over-voltage as well as limits negative voltage to around -0.5V which should be in the micro's safe area.

Keep going. You have the makings of a great project.

On choice of mosfet.... under full conduction at 30A, the 0.02R resistance will add about 18W worth of dissipation and what looks like 0.6V drop over the device. Not bad at all

1) put more in parallel, making it a great deal cooler ( by putting in parallel, you can reduce power dissipation

2) put more in parallel, making the total on resistance better

2) put a decent strip of heat-sink on the devices. Aluminium U-channel / whatever you have will be great. If you can use an aluminium enclosure, that will work as well. Remember to put a small fan on it to keep it cool. Don't waste too much energy.

Regards

John

zagene · Answer

Hi Everyone, I noted all of your comments and suggestions and this resulted a storm in my mind, thus my late reply. John, TY for your suggestions. Regarding the Ideal diode controller (mosfet driver), I've researched the component quite a lot in the past few weeks, designed some circuits etc but here is the problem: The the LTC4357 or LTC4352 is only locally available trough RS Components and id on my local currency extremely expensive although it is a very nice component and I would certainly like to build my designs around it. The footprint is also very fine and to etch a custom PCB and build it by hand would also present some difficulty. The price of it also make prototyping to expensive as I have learned that just merely calculating values and laying out a PCB do differ from real world conditions. So I just have to do without it (Im also not sure if the gate voltage would drive the available high current MOSFETs in saturation so that may be another hurdle making the design and layout more complex) As I went along with this design my ideas became more complex. My further question, and quite a nagging one, is would this prevent current flowback from the Battery and the "Charger" if I place a circuit like this between it. I recently lost a power supply that I used for battery maintenance charging at 13.5VDC because (it seems to be) the current flowed back into the PSU when the mains supply failed destroying the PSU or will I have to design another circuit similar to an ideal diode circuit to prevent this? My idea is to design a complete battery maintenance system that would utilize inputs from a PV charge controller (maybe an hybrid one for PV and Wind generator supply) to keep the battery charged with the input of the mains charger if the inputs from the DC charge controllers is too low, and thus using the battery system like a DC ups with inputs from mains supply and PV/Wind charger. I hope this make sense. We have a huge problem in South Africa with the mains supply (failing and not very constant level) and my (and fellow HAMS) main objective is to keep the communications network running if everything else fails. My knowledge is also limited to radio and electronic Hobbyist and DIY'er as I never had the chance to gain expert knowledge in anything. John, another thing, limiting the inrush current especially during the radio transmission periods which may require an instant current of up to 20A, would it be something similar like this circuit with Q2 as the 2N2222 2N2222 or must it be over the combination of the 2N2222 2N2222 and 2n7000 switch What would the influence be on the requirements of the radio during TX as if the supply current and voltage is not adequate it may damage the final PA and or not TX at all. Pardon for all this many questions Regards

michaelkellett · Answer

Forget the ZXGD3112 - it's totally weird, for example fig 4, page 6, needs two additional isolated floating 10V supplies and still ORs the source supplies via the MOSFET drain/source body diode. I can't quite decide if your circuit will work, when Input voltage is present U1 will conduct (due to the body diode) so the mid nodeof the MOSFETs will go up to nearly Input voltage. If Q2 is on the gate of U1 ispulled low so it turns on and shorts the body diode. So the source of U4 is at Input voltage and it's gate is low so it should turn on. I would reduce R2 to 10k and increase the 10Rs to 100R. To get a good idea of how it will work in practice why don't you use a simulator - LTSpice is free and can do this - don't worry about getting models for the MOSFETs - just use something similar that's already in the library. There are lots of other free simulators. MK

colporteur · Answer

Your post mentioned switching batteries. My commentary is not on the circuit but, on the batteries. My daughter has gutted and is retrofiring a 1983, 14ft travel trailer. I am currently building from the ground up, the electrical system, converter, fuse panel, wiring and battery. I have discovered that using 2 X 6VDC batteries to achieve 12VDC is preferred over a single 12VDC battery. The information I have is anecdotal. Both the travel trailer vendor and a battery supply house recommends 6VDC batteries used in golf carts. They indicated the performance of recover time, number of charge cycles and the length they hold a charge is much better. The 6VDC battery has a demanding market, that the battery companies are focusing on. The other benefit they suggest is the reduce cost of replacing a battery. I would think you would replace both batteries if you were doing a replacement but that is just me. I have taken them up on their suggestion to use 6VDC batteries. I am still working on the trailer so I can't give you any field data that suggests the recommendation has some merit. When you think about it, golf carts seem to have a lot of getup and go and last a few beers at least going around the course. Sean

Lead acid protection circuit

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