Battery max charge rate

Re: Battery max charge rate

If you have two or three parallel strings and design each string to pass the maximum planned current (discharging/charging), you should be OK with parallel connections (at least on the wiring side).

In larger computers, it is very common to parallel the wiring to carry more current than a single wire by itself can (limited space, using "cost effective" connectors, etc. usually limits us to relatively small awg wiring--and you can have a lot of current at 12v/5v/3.3v/etc.).

When I had to repair/redesign wiring and connectors that failed (charged connectors and/or wiring)--It was actually the "best connections/paths" that failed first. Which ever circuit path had the lowest resistance overheated first--Remember that Power=I²R. So self heating goes up with the square of the current. If you have two strings of batteries and one string has "a problem" (open cell, dirty/loose connection), the other string will carry 2x the "normal current" and the wiring will get 4x heating effect.

Battery current sharing is "stable" when discharging... I.e., when batteries get hot, their output voltage falls a bit, and tends to "unload" (output less current into the shared load).

However, battery current sharing is somewhat "unstable" when charging under heavy current. When the same battery (or battery string) gets hotter than the rest, its voltage falls a bit, and it wants to accept more current than these rest of the strings. Which gets it hotter, more current, etc.

So, my caveat is--If you decide to have parallel strings of batteries, make sure that you have heavy/balanced wiring to keep currents balanced as much as practical--And you should have a fuse/breaker per string if you have parallel strings to protect against short circuits.

And, I would highly suggest that a person gets an DC Current Clamp DMM (again, here is a cheap one that works "good enough") and monitor the current in each parallel battery string and look for unbalanced current during heavy charging/discharging. If you see >50% spread between the high current and low current string, you may have a problem that needs to be addressed.

I can understand people wanting two parallel strings for redundancy. I still believe that lots of paralleled strings are a pain in the rear end to maintain (watering, checking connections, etc.). And that, at least initially, if you follow my recommendations about heavy wiring and fuse/breaker per string, are not cheap to install.

But, I do agree, that if you need/want to parallel your battery strings--It is not the end of the world either. It is just more work to maintain.

I believe the Chris said that he disconnects his banks and rotates the re-distributes the batteries physically in the strings... That is a lot of work, and for him, seems to be working well to do that.

We have have more than a few posters here that say they seem to have batteries that fail in one position in their strings more often than others--And they shuffle batteries too.

Just remember to use insulated tools (tape on wrench handles) and were safety goggles/old cloths when doing this. Batteries can be dangerous if miss-handled.

-Bill

Re: Battery max charge rate

I should also add--That many folks tend to think that massively paralleling devices together makes for a more reliable system.

In general, it does not. It actually can make for a less reliable (over all) system). You have 4x as much of anything, the over all chances of failure are 4x as much.

Yes, if you have two parallel strings and one fails for some reason (open/shorted cell), you can limp along with one string until you get a replacement. However--An open or shorted cell can be missed if people are not doing performing inspections (using a DMM to measure the voltage across each cell/battery in a bank, using an DC clamp meter to measure charging/discharging current, etc.).

One of the major headaches with parallel redundancy (N+1 in the engineering world, N is the number needed operate, +1 is the spare). You need to detect when one member of "N" fails and fix it--As that failed member can damage others in the cluster or you have have several N's fail, and not have the redundancy when needed.

A poster here went another method for their redundancy... He had a 48 volt battery bank made from 2 volt cells. When one cell failed, he simply made a 46 volt battery bank and cranked back the charger by ~2 volt. Everything worked well enough that he could operate normally until he received a replacement cells in a couple weeks (or months?).

Which brings up another issue of going with "high voltage" battery banks. You have a 12 volt battery bank and one cell fails--Your bank is at 10 volts nominal and nothing works. (10.5 volts is cutoff for a typical inverter).

You have a 48 volt battery bank and one cell fails, you have 46 volts and things still work sort of OK (42 volts is cutoff). But there is the possibility of that shorted cell overheating or discharging parallel connected strings (in a redundant power system, we would usually use diodes or FETs to isolate the outputs in case a supply shorted internally).

With parallel strings, proper inspections and maintenance becomes even more important to ensure that you have high reliability and good overall battery bank life.

-Bill

Re: Battery max charge rate

ChrisOlson wrote: »

Same thing in airplanes. A twin-engine aircraft has 2x more chance of having an engine failure than a single engine. But you have 10x better chance of surviving it without a crash.
--
Chris

This is why engineers are boring people at parties and tend to avoid bars:

Why Twins Crash

Mostly it's the usual suspects: fuel, weather and runway prangs. But poor maintenance is a surprisingly big player, as Aviation Consumer reported.

Between 1972 and 1976, the NTSB investigated the outcome of twin-engine crashes and concluded that in the event of an engine failure that resulted in a crash, the likelihood of it being fatal was four times greater than a crash in a single. (Despite the paucity of current data, it's still a lead-pipe cinch that a single with a failed engine is coming down somewhere. It may or may not result in a crash.)

Our most recent review of twin accident data tends to support this finding in general, if not to the same decimal point. Typically, in crashes where an engine failed in a twin, the accident was fatal between 20 and 50 percent of the time; in singles, the fatal rate for powerplant failure is lower, on the order of 10 percent, somewhat variable with model.

Why this is so is not too difficult to understand. Bluntly, pilots sometimes screw up engine-out emergency procedures by reacting too late to the developing situation or simply losing control. In some cases, they expose themselves to certain disaster by expecting too much of the airplane's single-engine performance or pushing the weight and balance envelope to the breaking point.

In one accident, a Baron was loaded 800 pounds over gross weight and couldn't maintain altitude when one engine failed. No surprise there. It crashed, killing all six aboard. It didn't help that the pilot had a fresh multi-engine rating and little experience in the airplane.

The answer is "it depends"... But conventional wisdom is not always correct for the "simple answers".

We have had several folks here with shorted cells that did damage series connected cells (they over charged) and killed bank (discharge the rest of the bank).

Granted, there should be lots of warnings--But those warnings (poor performance, one or more hot batteries, measured voltages/current flows, etc.) have to be observed in order to be useful.

One of the common symptoms of battery fires in large banks (central offices, backup power systems) is when the batteries start drawing >~2% of their rated current when they are full. It is not always a lead bar across the plates--It is something else that slowly affects overall battery/cell function.

Read the rest of the article... It is not that twins are more (or less) reliable/fault tolerant that is "killing people"--It is they are more complex. More can go wrong and pilots have additional chances for error. Simple problems/failures eventually lead to the site of the crash.

And back to why I try to keep off grid power systems as "simple as practicable". If not well maintained and inspected--The most complex/redundant system will still fail when needed--And is actually more likely to fail than the guy who has a trolling motor battery and a 300 watt MSW inverter in the garage on a trickle charger.

-Bill