Schottky diode-or not sharing load / uses lowest voltage battery?! - Page 1
Forum section: electronics / projects

author: RandomJoe
author_rank: Newbie
timestamp: July 30, 2022, 01:11:07 am
content: I'm working on a simple arrangement to power some astrophotography gear from a pair of Ryobi 40V batteries (one is sufficient to run the rig, want two for runtime / be able to swap in a fresh battery without shutting down). I bought a couple of STPS240H100TV1Y (datasheet attached) "automotive 100V, 2 x 120A, power Shottky rectifier" devices as they have two diodes in a single package. In testing I've found a very odd behavior that I don't understand! (But then maybe I've forgotten something - my electronics classes were a long time ago!)The setup is two Ryobi battery adapters, each connected to one of the diode anodes. The cathodes are tied together and feed the 13.8V regulator powering the astro rig.What I expect to happen is the higher-voltage battery would take the load, until both batteries were about equal, at which point they would share the load. I also expect that I could disconnect either battery at any time and the load is instantly picked up by the other battery.I do see that in certain circumstances but fairly often I find that the *lower* voltage battery has taken *all* the load with no current being drawn from the higher-voltage one. When this happens, if I unplug the lower-voltage battery (that's supplying the load) there is a very brief drop in output voltage (frequently enough to cause the connected load to reset) before the second battery picks up the load.This does seem to be current-dependent to some extent. When I tried this with the 40V Ryobi batteries and the astro computer I was drawing about 500mA total. When I tried testing with just an LED light the current was down around 200mA and the load-sharing worked exactly as expected. I then switched to 18V batteries with the light which brought the current back up to 400-450mA and once again could reliably get the diodes to "stick" on the lowest-voltage battery.I can manually cause this to happen by connecting everything, then unplug/reconnect the higher-voltage battery. I will occasionally see the input voltage to the power supply rise back up to the higher battery voltage, but usually it stays at the lower. Unplug the lower-voltage battery and the input voltage briefly drops (have seen 7V on the meter) before going up to the voltage of the remaining battery.However, it will also happen on its own - whether the batteries are at roughly equal voltage and sharing the load, or one is higher than the other and currently taking the load, at some point all load switches to the lower-voltage battery!So am I forgetting some esoteric property of Schottky diodes, or is there something peculiar about the ones I bought? (I got two of them, so also tried using just one diode in each package in case it was a side effect of being both in one package, same behavior.)Thanks!

author: Weston
author_rank: Regular Contributor
timestamp: July 30, 2022, 01:54:28 am
content: Your intuition seems correct. One slight note is that large schottky diodes are very leaky. You are possibly getting a few mA of reverse current though the diode. However, that should not impact anything.Have you checked the terminal voltage of the higher voltage battery when the lower voltage battery is taking the load? It sounds like there might be a BMS circuit or something similar disconnecting the higher voltage battery.I found a thread online about the Ryobi 40V battery pack and the BMS seems pretty fancyhttps://www.diyelectriccar.com/threads/ryobi-40v-bms-troubleshooting-and-repair.197409/I did not read through the thread the entire way, but could you be having issues with inrush current when plugging in the new battery triggering some protection in the BMS. Or triggering something relating to the charging circuitry?Do you have a lot of capacitance in your system? Do you hear a pop when you plug in a fresh battery?

author: RandomJoe
author_rank: Newbie
timestamp: July 30, 2022, 02:37:47 am
content: I still measure the full voltage on the higher battery when this happens, so I don't think the BMS is cutting the output. Ryobi's BMSes do seem to be very touchy though, so maybe it's doing something squirrelly. Among other things, it's way too easy to trip the overcurrent protection with some of their brushless tools. When this happens, the output goes to zero (at least the tool stops and any lights go out) but as soon as you release the trigger the battery resets and you can immediately restart. Hm, maybe in this case the BMS is just constantly triggering over and over very fast? I'll check with a scope and see if there's any fluctuation in the battery voltage.There is apparently a fairly large capacitor in the power supply, as I do get a small but visible / audible spark when connecting the first battery. However everything works properly on first connection, it's only after I've disconnected / reconnected a battery or if I leave it running on its own for a while that this happens.It happens most often when "swapping" (unplug/replug) a battery so the inrush would be considerably lower since just (at this point) a 1-2V difference between my batteries since one is charged and the other only slightly discharged.It has also happened a few times while both batteries were within a couple tenths of each other and sharing the load fairly equally. I measured the individual battery currents with a DC clamp meter and they were roughly the same. Left things running for a few minutes, then checked again and found one battery supplying all the current.

author: Peabody
author_rank: Super Contributor
timestamp: July 30, 2022, 04:23:34 am
content: The forward voltage of a schottky diode is reduced as it warms up. And the diode passing the most current will heat up the most. So the battery carrying the load could have a lower voltage than the idle battery, but the voltages after the Vf of the diodes could be the opposite. Could this possibly explain what you're seeing? Have you tried this with normal rectifier diodes, just as a test, to see if the same thing happens?

author: RandomJoe
author_rank: Newbie
timestamp: July 30, 2022, 01:39:35 pm
content: Oh, the answer seems so obvious in hindsight...I have also just had another fine lesson in "you can never have enough meters"!Turns out the (not actually a) "problem" is in fact the BMS in the batteries.Last night I did NOT have enough meters. I was working basically with things sitting in my lap as I leaned over my astro rig's control box. I had two of these Watts Up inline voltage / power meters (https://www.sparkfun.com/products/retired/14331) and was moving them around in various locations to see what was going on. This effectively hid what was happening from me.This morning I cleared off some bench space and laid everything out with a DMM on each battery and a third on the power supply input, as well as the two Watts Up meters. I quickly saw that the second battery connected does NOT go to full battery voltage, it just gets *close*. On further testing I realized that Ryobi batteries don't actually always have the output turned on, for instance a charged battery will be about 20.2V when in use but with no load attached the terminal voltage is only around 19.8V.I don't see this with the Watts Up meters because their own load (about 7mA per the datasheet) is enough to turn the battery output on! Thus, last night, everything would work perfectly when I had the Watts Up meters inline with the batteries. When I removed them the second-connected battery would never turn on its output.Now I guess I just need to determine the minimum load required to turn on the output, and make sure I have that.I did also try some silicon rectifier diodes this morning, of course the same thing happened since it was the BMS, but I did get to see one effect of the difference in reverse leakage. With the schottky diodes, with one battery removed the unconnected diode's voltage would slowly rise to 16-18V while with the silicon diodes it stayed below 1V.

author: RandomJoe
author_rank: Newbie
timestamp: July 30, 2022, 03:51:01 pm
content: More testing with a variety of resistors and the Ryobi batteries don't need much of a load to keep the output turned on.A 100k resistor across the 18V battery will keep it active, with 1M the output turns on but after a short time goes back off.Surprisingly, the 40V battery will go active with even higher resistance, it'll stay active with just the input impedance of my Fluke 116. The 121GW will also turn the output on, but it occasionally drops out for a fraction of a second so apparently not *quite* enough to keep it happy.Considering my load will be a fairly constant 0.5-1A, I suppose I can tolerate an extra 0.0008A from placing a couple 100k resistors across the batteries to keep them active...