Question about over-charging Lead Acid batteries - Page 1
Forum section: electronics / general

author: globoy
author_rank: Frequent Contributor
timestamp: July 26, 2022, 04:46:26 pm
content: Like many of you, I'm sure, friends come to me (an EE) with their technical questions. I recently had a boat owning friend come to me with questions about charging the lead acid batteries on her boat that I'm unsure of an answer for. Currently the boat is docked far away for a few months and she's a little concerned she is over-charging the batteries. I am hoping some of you can shed some light.The boat has a ~700 AHr @ 12V battery system (two parallel strings of two six-volt batteries in series). Currently two independent 3-state (Bulk, Absorption, Float) chargers are attached to the common battery rail. A line-powered unit and a small (<100W) solar powered unit. The boat owners thought that would be good backup in case power failed. There is probably around a 1 A continuous load on the batteries (solar charger burden + some monitoring electronics).Before they left the boat a couple of thousand miles away they observed the following: The line-powered charger seems to remain in float charge state all the time. But during the day the solar charger goes into Bulk charge mode when the sun starts hitting the panel and the battery voltage rises to about 14.4 volts. The solar charger is capable of up to 10A charge current but shows about 1A charge current during this time. The charger doesn't exit Bulk/Absorption charge state until the sun fades and then the battery voltage falls back down to the ~13.6-13.7 volt float level.Now that they are home, my friend is worried that the daily increase in voltage over the float level might damage the batteries. I did some reading about over-charging lead acid batteries and there is talk about corrosion of the positive plate and out-gassing due to increased temperature. However the articles I read said that corrosion occurs after extended periods of too-high voltage (> 48 hours). In addition it seems that a 1A charge current into set of batteries with a total of 700 AHr capacity wouldn't really heat them up too much (it is hot there though during the day).Might there be a risk to the battery life with this situation? They don't plan on returning to the boat for another 6-8 weeks.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 26, 2022, 05:55:53 pm
content: I still have faded diary notes from long ago when I was on telecom rectifiers.The battery will be on the edge of H2 liberation at anything over 2.30 V/cell float.Some refs say gassing is occurring at 2.35 VHere is confirmation from a randomly selected battery vendor:https://support.rollsbattery.com/en/support/solutions/articles/816-hydrogen-off-gas-ventilationIn view of the cost of the batteries, it should be prudent just to ask the marina person to disconnect the solar. (?)

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2022, 06:06:57 pm
content: The exact answer would depend on the exact cell chemistry (there are variants) and the temperature of the batteries. Considering that there is a continuous load on the batteries and the charging current is only 1A during the day (I'm presuming the float charger adds nothing at that point, which is a presumption that bears examining when possible) I seriously doubt that there will be any significant degradation of the batteries in the next few months. When they get there, they can check the electrolyte and if they need water, then perhaps make some adjustments. I'm a little surprised that 1 amp brings a 700AH battery up to 14.4V. Do you know what the approximate ambient temperature is?

author: langwadt
author_rank: Super Contributor
timestamp: July 26, 2022, 06:08:24 pm
content: Quote from: mag_therm on July 26, 2022, 05:55:53 pmput a relay in series with the solar charger, controlled by incoming power so it is only using solar in case of a power failure?

author: trobbins
author_rank: Frequent Contributor
timestamp: July 26, 2022, 11:59:14 pm
content: The actual brand and model of battery is likely quite relevant, as well as its age (assuming all 6V monoblocs were installed new), including any general operating instructions for that battery model from the manufacturer. The battery will have a self-discharge rate that could easily be in the ball-park of the external idle load (given that is circa 1A). I'd suggest the solar recharge rate is too minor to be of any consequence (assuming the battery is circa 700AHr at >24hr discharge rate, as that would indicate a daily charge of circa 7Ahr or 1%, and could be even less if the 700AHr was at a C/10 rate).At some point in the future, the battery should be tested for load hold-up, but that may be logistically difficult to perform, as it likely involves disconnecting both chargers and monitoring battery voltage over a period of time, and then reconnecting the mains charger to confirm appropriate recharge to float conditions. That test is preferably done with current and voltage sampling measurements, and some estimate of battery depth of discharge to give confidence that the monoblocs are showing nominal operation.

author: gnuarm
author_rank: Super Contributor
timestamp: July 27, 2022, 05:31:01 am
content: I have always found batteries to be the most complicated part of any system. They are finicky bi***es, never being happy with what you feed them.That being said, I worked for the railroad for a while and they had lead acids on trickle charge with minimal use (flashing the crossing lights and working the gates a couple of times a day) for 20 years!I'm concerned about hanging two chargers on the batteries. That just doesn't make sense to me. Are they worried about the power being off for weeks? 700 Ahr would provide nearly 30 days of power for a 1 amp load. That's a major power outage.I would leave off the solar charger and see what the standard charger does during the day. If it can keep up with the 1 amp load when trickle charging, I don't expect it would leave trickle charge mode. 1 amp is a large trickle charge though. If the 1 amp load makes the battery voltage drop enough to enter full charge mode once a day, that's not too bad. The batteries probably will only last 20 years though.

author: globoy
author_rank: Frequent Contributor
timestamp: July 27, 2022, 08:34:02 pm
content: Thanks, everybody, for your answers. I appreciate it.Quote from: bdunham7 on July 26, 2022, 06:06:57 pmI was surprised by this as well. It's possible something else is going on and went un-noticed or un-remembered. The boat is docked near Washington, DC so I suppose ambient temperature is reasonably high at this point in the summer.

author: gnuarm
author_rank: Super Contributor
timestamp: July 27, 2022, 09:14:48 pm
content: Quote from: globoy on July 27, 2022, 08:34:02 pmYeah, temps are definitely high around DC at the moment.Looking at various references, the 1A charging current is in the ballpark for trickle charging. But it should be done as a constant voltage source, rather than a current. I assume the 1A is what you are measuring rather than what the unit is set to provide? Standby charging should be voltage output.I found a source for this at Battery University which has tons of good information.https://batteryuniversity.com/article/bu-403-charging-lead-acid"A warmer ambient requires a slightly lower voltage threshold and a colder temperature prefers a higher setting."So your hot temps should push the float voltage DOWN, not up! So if your average battery temp is, say, 27°C, that would be 2.3V per cell or 13.8V. But at very high temps, like right in the sun at 40°C, it would be more like 2.25V per cell or 13.5V.https://batteryuniversity.com/article/bu-410-charging-at-high-and-low-temperaturesI would say your 14.4 float voltage is far too high. That is more like the constant voltage "topping" charge done before the float charge. The end of topping charge is triggered by the charging current dropping below a level. If the battery has a load on it while being charged, the charging current may never drop below this threshold to end the topping charge.What I find odd is that there are two strings of 6V batteries, that are charged together, rather than being charged separately. It would seem a better approach would be to charge one string, while the other is being used. They get switched when the battery string being used drops to a threshold. Or maybe a combination of one being fully charged, and the other having dropped to at least 50% discharged. This would allow each battery string to be charged as needed, with no load connected to mess up the charging control.

author: boB
author_rank: Frequent Contributor
timestamp: July 27, 2022, 09:19:50 pm
content: You said "corrosion"... Did you mean sulphation (sulfation) of the battery plates ?Salt water causes corrosion of course...Sulfation is normally caused by too low of charge voltage for too long of a time.Depending on ambient temperature around the FLA batteries, 14.4V isn't too high.The charger should really have temperature compensated absorb voltage. Too high of absorb voltage will cause the water in the battery to boil away so you will need to check the level and add to it when dry. But too high of voltage for too long can also cause the batteries to degrade faster just as putting amp-hours in and taking amp-hours out does.As always, use the battery manufacturer's recommended charging techniquesboB

author: Zenith
author_rank: Frequent Contributor
timestamp: July 27, 2022, 10:22:42 pm
content: It doesn't seem like a great idea to have two battery chargers working independently, with the solar powered one pitching in with enthusiasm when the sun shines. The float voltage goes down with increasing temperature. Float chargers supply just enough current to counter self-discharge at the float voltage level. There are chips for charging SLAs at least, and they have temperature sensors. I wonder if the two chargers interfere with each others action.I'd be inclined to disconnect the solar charger on the assumption that line power cuts aren't that likely.It's been a long time since I've done anything with lead acid batteries. Someone else said checkout Battery University.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 27, 2022, 11:04:20 pm
content: The voltage at each gasing starts to happen depends a lot with the temperature.Code:Lead Acid generic specs-----------------------At a comfortable temperature of 20°C (68°F), gassing starts at charge voltage of 2.415V/cell. When going to 20°C (0°F), the gassing threshold rises to 2.97V/cell.Battery status-40°C (-40°F)-20°C (-4°F)0°C (32°F)25°C (77°F)40°C (104°F)Voltage limit on recharge17.116.215.314.714.1Float voltage at full charge15.314.714.113.813.5or loweror loweror loweror loweror lowergasing at 20*C for V >= 14.4Vnominal, no load = 12.6Vfloating charge = 13.5...13.8V

author: gnuarm
author_rank: Super Contributor
timestamp: July 27, 2022, 11:42:16 pm
content: Quote from: RoGeorge on July 27, 2022, 11:04:20 pmThe thing that has always confounded me with battery charger design, is that the data supporting the various numbers seems to be in short supply. It seems to be one of those things where one company does a test to set the voltages and they keep using that data for the next 50 years. Another company, making very similar batteries, will have some different numbers, and they use that for 50 years.It seems like every source provides different numbers and very, very few provide the background data to support those numbers. At least, they don't vary much, but it doesn't take much change to be overcharging a battery. I've seen circuits that had more variance in output voltage because of component tolerance, which means it nearly doesn't matter which set of numbers is your target. I've even see circuits that measure the temperature, with a probe inside the charger, rather than in contact with the battery! I guess they figure it's better to measure the wrong temperature than none at all?

author: Zenith
author_rank: Frequent Contributor
timestamp: July 28, 2022, 12:37:27 am
content: If the probe is inside the charger, there's a possibility of it being warmed by the circuitry, but with a bit of thought it could be roughly ambient and about the same as the battery - in most cases. If the probe was in contact with the battery, it implies a lead and some way of attaching it. That's something to break off or be left dangling. There could be a probe in a stand the battery sits on, but that's got obvious problems such as the battery having to be charged in place.The sensor in the charger is probably as good as it gets easily. I've seen chargers with a cold setting for temperatures near and below zero.I'd guess that charger designers do some research and also check the literature so as to come up with a good enough design, which works in most cases. Battery overcharging doesn't seem to be a big problem in general.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 28, 2022, 01:07:09 am
content: I'm surprised to hear that your friends have to worry about remotely maintaining their boat in the DC area. My family keeps a boat in a boat storage yard (actually inside of a multi story building) directly on the Potomac River in Arlington just outside of DC and the workers there keep the batteries charged and maintain the boat and put it in the water when we want to use it and then take it out and store it when we're finished. Keeping a boat in the water there is rare due to the weather and the very limited dock space available and the extremely high resulting costs. Yes, it is dammed expensive (everything is Expensive in the DC area!) but it's a complete tax write off for one of my family members.I agree with some of the other respondents, if AC power is available use that to charge the batteries and disconnect the solar system. Trying to use two charger systems simultaneously is probably going to result in neither one working correctly.

author: gnuarm
author_rank: Super Contributor
timestamp: July 28, 2022, 01:43:18 am
content: Quote from: Zenith on July 28, 2022, 12:37:27 amThat's the problem. They take a lot of shortcuts with the result of the charging not being very ideal with a shorter battery life. Auto use is a perfect example. Some cars charge the battery at 14.5 to 15 volts at all times the engine is running. No attempt is made to accommodate the charge cycle of lead acid batteries. So many fail after just 3 years.

author: Zenith
author_rank: Frequent Contributor
timestamp: July 28, 2022, 01:34:25 pm
content: Quote from: gnuarm on July 28, 2022, 01:43:18 amI've found auto batteries typically last ten years. The line powered chargers are typically crude, but most people don't need to use them regularly. What they won't tolerate is deep discharge, say by the lights being left on overnight. People who only use the car for short journeys get through car batteries; three miles to work with the headlights on, and let the car go cold over eight hours, then the same again home, and the car's charging system just can't keep the battery properly charged. Also those with auto batteries not used for long periods such as in vintage cars, or motorcycles left over winter, have serious problems because of self-discharge and sulphation, with a new battery needed every spring. They can avoid that with a float charger, which just counters the self-discharge.A few years back there were halogen lanterns and bicycle light systems with SLAs. The batteries in those didn't last long. The chargers were extremely crude, and a proper charger would have cost at least as much as the product, without adding any obvious advantage at purchase time for most people. The lantern needed to be never left in a partially discharged state, and that wasn't a discipline that most users could be expected to follow. The battery wouldn't tolerate being heavily overcharged either, and the crappy chargers they came with would certainly overcharge them.I'm sure globoy's problem would be improved by using one charger or the other, not both, and the line powered charger seems like the best bet. Whether the line powered charger is sophisticated and can keep the battery in tip top condition allowing for temperature, is another question.

author: globoy
author_rank: Frequent Contributor
timestamp: July 28, 2022, 07:11:28 pm
content: Quote from: Zenith on July 28, 2022, 01:34:25 pmYeah, I'm going to suggest they disconnect the solar charger and only use that when AC power isn't available where they are. Supposedly both chargers are "very" smart. The line-powered charger is also an inverter and has a ton of settings that the owners set according to the battery spec sheet. I'm not sure how many parameters the solar charger has.The problem for them is that the boat is 2000 miles away so they have to either find someone to do this for them or fly out before they return in 6-8 weeks (or just live with the situation and hope the batteries aren't being damaged).

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2022, 07:27:03 pm
content: Quote from: globoy on July 28, 2022, 07:11:28 pmI seriously doubt there would be any damage in that timeframe. I think at the most they'd need a bit of water. In a boat sitting on the water and with their large thermal mass, I doubt that the actual battery temperature is getting much over 80F unless something is different than what you stated so far. Also, a lot depends on the exact battery used. If it is a calcium-cycle battery, there's no concern at all. If it is a standard antimony type deep cycle, then they'll just want to check the water when they get there and make adjustments accordingly.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 29, 2022, 12:00:37 am
content: Imho, globoy has only provided scant technical details so far - so a few comments to date seem as scant in technical credibility.The actual battery type (apart from being lead-acid) and make/model is not defined, or its age and history. The two charger systems are poorly defined - no identification of mains charger's current capability or actual overnight float current (or a breakdown of load and battery current), or temp-comp, and only a measurement of 13.6-13.7V - no identification of PV supply capability or regulator make/model, only that the regulator has a 10A capability and only a measurement of 1A output (with no comment on whether that is a spot measurement or the solar conditions). The mains availability and history of blackouts is not commented on.I can't see any technical justification to change what exists - especially any recommendation to disconnect the solar charger - that just sounds like a knee-jerk reaction.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 29, 2022, 02:55:14 am
content: Quote from: trobbins on July 29, 2022, 12:00:37 amI would go the other way---use the Solar charger, & when its output falls, auto switch to the line powered one.Obviously, that will imply extra complication in both devices & their connections, so maybe not "doable" in the OP's case.

author: globoy
author_rank: Frequent Contributor
timestamp: July 30, 2022, 04:51:31 pm
content: Thanks again everybody. I shared the various answers with the boat owners.Regarding lack of specificity - sorry, I had only what information I was provided over dinner by people who aren't as technically minded as the ones in here :-)