Float charging - Page 1
Forum section: electronics / general

author: SirNick
author_rank: Frequent Contributor
timestamp: July 16, 2014, 07:28:59 pm
content: After reading through the hybrid rant thread, I've started thinking about vehicle battery maintenance. I don't own a hybrid, but I do have three vehicles -- a motorcycle, an old VW Ghia, and a 2012 Tacoma. I also keep a spare lead-acid battery (it came with the Ghia) that I can use as a swap-out, or for a jump start if need be. Winters here are serious business, so the bike and Ghia only see the road in the summer. I sometimes exchange the Ghia's battery with the spare just to balance their wear and tear.So, four batteries to maintain -- several that sit for months at a time. What maintenance should I be doing? I am willing to build dedicated chargers for each.If I get the gist from the hybrid thread (regarding traditional lead-acid batteries), a high charge voltage (~14.5v) for long periods of time will harm a battery, but may be necessary occasionally to prevent sulfation. A more reasonable voltage (13.8v) will prevent self-discharge and/or draining from quiescent loads.What would be an ideal pattern, then? Maybe a constant 13.8v, with 14.5v for one hour a day? (Less? More?) Is this sufficient, or would there be a benefit to special treatment? E.g., initial CC charge, or what to do with discharged batteries? Is it perfectly OK to keep the car's electrical system connected? (That would make it easier -- merely a quick disconnect for in-season use.)I have read a few articles on this sort of thing, but I'm interested in what a group of engineers has to say. There are a lot of opinions out there, and some of it depends on usage patterns.

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 16, 2014, 07:46:28 pm
content: The best option if the cars are just standing is simply to float charge them continuously.However, if there are going to be large temperature swings then you should have a temperature compensated float voltage type charger. The float voltage of a lead-acid battery has approx. a -4mV/deg C per cell temperature coefficient. That means 24mV drop in float voltage for every degree Celcius increase of electrolyte temperature.The easiest way to measure the battery temperature is to connect a sensor onto one of the terminals.If the battery is not going to be discharged at all then a periodic equalization charge is not required.The charge regime described above is used on virtually all commercial high current industrial lead-acid battery systems (such as telecoms sites, telephone exchanges etc).

author: Legit-Design
author_rank: Frequent Contributor
timestamp: July 16, 2014, 07:59:24 pm
content: Any charging and keeping the battery charged up is better than not charging it and letting it to sulfate. Whenever lead-acid battery is not fully charged it will start to sulfate. Lead sulfate crystals start to form on the plates and surface area will decrease. And that will become more severe if not charged up immediately (that is what I've read and it seems to make sense) Driving short trips is the worst thing for any type of car, especially in the cold. Usually batteries in cars don't live very long because they are not kept fully charged and are left to sulfate, then winter comes and this sulfated battery that could before barely start the car is just dead and cannot start the car.It's ok to give lead-acid battery some balancing/desulfating charge if you keep eye on the liquid level inside as it tends to lose more water and will get ruined if the plates will be exposed to air.Also those maintenance free lead-acid batteries have just better recirculation systems for the gasses, but they might still lose water as vapor. So locating the filler caps under a sticker usually and keeping the level safely over the plates is a good idea. Just be careful with that battery acid, you don't want it on anything that you don't want to get ruined, including your skin. Never use anything else to fill up the battery except battery water. Usually there are marks inside the cells where the acid level should be at. If your battery is not fully charged don't add water to it, charge it first and then add the water.

author: SirNick
author_rank: Frequent Contributor
timestamp: July 16, 2014, 09:18:34 pm
content: OK, so what's the ideal reference voltage and temp to use for derating? 13.8v at 25C?EDIT: NM, Googled that all by myself.Looks like 13.4v at 25C.

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: July 16, 2014, 11:06:48 pm
content: I thought it was 13.6v? I know when I take my bike off the charger, it's 54.4v (13.6x4).

author: xygor
author_rank: Regular Contributor
timestamp: July 16, 2014, 11:45:27 pm
content: You might want to look into something like these::http://www.amazon.com/Optima-150-40008-Battery-Charger/dp/B00DHGPVKI/http://www.amazon.com/NOCO-G7200-Battery-Charger-Maintainer/dp/B004LWTHP2/The Noco one is also available for 220V. Not sure about the Optima.

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2014, 12:00:13 am
content: I use 2 rules for maintenance charging of auto batteries.Run vehicle or charge at least monthly.Vehicle charging systems regulatemaxat 13.8-14.2 V, so I base any charging decisions on that.Check a stabilized fully charged battery voltage and you will see ~2.2 V/cell.For maintenance only, I would like to see just above 13.2 VOr get Optima batteries or similar that can stand long no use periods.

author: SirNick
author_rank: Frequent Contributor
timestamp: July 17, 2014, 01:03:51 am
content: According to Wikipedia: 13.05v (2.18v per cell) for gel, 13.4v (2.23v / cell) for flooded lead-acid, and 13.6v (2.27v) for AGM.So here's what I've learned from these threads and other sources on the Internet. Most of it agrees, more or less, but I've run across some differences as well. If you know a thing or two about lead-acid battery maintenance, chime in.The first thing you would want to do is get the open-circuit voltage and temp from the battery. If the temp is 50C or above, stop. (Conversely, the electrolyte will freeze at -9C at ~40% charge, but should survive down to -68C if 100% charged. Another source quotes freezing at 0C for a discharged battery, and -40C for a charged battery. No idea if those are referring to equivalent percentages of charge.)If the battery voltage is 2.1v per cell or less, it needs to be charged or else sulfation will occur (or has already begun.)Here's where things get dicey. It appears the "ideal" charging profile is based on percentage of the battery's capacity in amp-hours. (FYI: This rating is actually measured with a load that will result in 20 hours of runtime until the battery is considered dead -- ~1.75v per cell. The result is then scaled to the average amps per hour.) However, it's probably safe to assume that this figure won't be known to the charger ... and that age will affect this value anyway. So, from what I can tell, a reasonable compromise is to do this:If the voltage is less then 2.3v per cell, go into constant-current mode (a reasonable charging voltage would be 2.15 to 2.5v per cell) until the battery begins to limit the current drawn. This is the end of the "bulk charge" cycle, and the beginning of the "topping charge" cycle. Up until this point, you can dump a nearly infinite amount of current into the battery (one source says 20% of Ah rating is optimal), but once the current begins to fall, you have to be careful, as anything over ~2.3-2.4v per cell will cause gassing. If the cells don't get to this voltage within 24 hours, they never will.Continue at 2.3-2.4v per cell until the current falls to 2% of the battery's Ah rating -- which, again, we probably won't know, so alternatively, maintain constant-current until the cell voltage reaches 2.3v. If this doesn't happen within 10 hours, the battery is probably shot. This is the end of the "topping charge" cycle, and we are on to the "float charge" cycle.Switch to constant voltage mode. Apply the -3.9mV per cell (-24mV for a 12v battery) per degree C derating curve to the optimal float charge level, depending on the battery design. This can go on indefinitely, but charging can stop now if the battery is being put in storage with no load on it. Re-check every 2 to 6 months, depending on storage temperature. (Over 25C, every couple months. Around 20C, twice a year.)Alternatively, the topping charge cycle can be extended to deliberately over-charge the battery as part of a cell equalization process. The cell voltage will reach ~2.5v (sealed) to 2.6v (flooded) in constant-current mode, and should stop rising. This is as full as the battery will get, and going any further is just going to gas off the water.Comments?

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2014, 01:36:17 am
content: QuoteThis process can be useful in my experience although I resist using it on newish batteries.I have seen improvements after repetitive bench charge/discharge cycles of lead acid batteries.I experimented with high energy pulse de-sulphating circuits years ago with some success.Amazing to watch battery voltage lift after a circuit thatdrawscurrent has been placed on it for a while and battery performance improve also.But you can't save them all.

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2014, 02:01:26 am
content: QuoteThe widely used rule ofisbattery capacity (A/Hr) divided by 10= max charge rate in Amps

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 17, 2014, 02:38:28 am
content: How about another strategy. Get a regular automatic multistage charger and have it come on once a month to charge the batteries. If you wanted to just use one you could connect it to each battery in turn with say a relay (though possibly you could do it with diodes like a "battery isolator", though the drop might mess up the charging alogrithm).Advantages I see are that you're only powering the timer when idle, so that could be very small current consumption. Also charging batteries this way is better than trickle for the battery.Incidentally I noticed that this is the strategy used in newer, energy efficient emergency lights. The side benefit is the batteries last several years longer.If you do trickle, you can get around the water loss by topping up the batteries. All the automotive ones I've seen have removable caps, although they don't usually make it obvious.

author: johansen
author_rank: Super Contributor
timestamp: July 17, 2014, 02:46:06 am
content: there exists a solution: constant current charging.yes, you have to top up the battery either way.if the current exceeds the self discharge current, it will boil away water.if the float voltage is too high, it will boil away water.same difference attempting to calculate it.as far as i know the self discharge current should be more predictable than the float voltage, as a function of temperature.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 17, 2014, 03:04:38 am
content: Charge to 14.4V for something like 15-30 minutes per day.

author: SirNick
author_rank: Frequent Contributor
timestamp: July 17, 2014, 04:05:08 am
content: I own a (fairly cheap) battery tender already. Although, I end up using as a slow (but highly portable!) batterychargerwhen I inevitably forget to use it on the off-season batteries. So, I'm looking at this from two angles...First, whatshouldI be doing, technically, and does my Black and Decker cord lump do that thing reasonably well?And, second, would this be a fun and feasible project, where I might even be able to build a device that can stay nestled somewhere on or in each vehicle... or would I be more likely to cover myself and half my garage with boiling battery acid?So far, it looks like this is probably within my level of skill. That is, lead-acid batteries appear to be resilient enough to minor abuse that I would probably not have to make humbling calls to my insurance agent(s), and not so complicated that building a microcontroller-based charger is something I would only do if I intended to make a career out of battery charging devices.Now, if this were Li-Ion, I would just use the IC that manages this stuff, then plug it in and run like hell.

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2014, 04:29:03 am
content: QuoteLooked high and low in my archives for a simple charge controller that I could recommend, but darned if I could find it.It just passes what current you can provide until the adjustable trigger voltage is met.SCR and UJT based and 10 turn preset based.I'll have a hunt for it.Found much the same:http://www.google.co.nz/imgres?imgurl=http%3A%2F%2Fwww.next.gr%2Fuploads%2F135-9654.png&imgrefurl=http%3A%2F%2Fwww.next.gr%2Fpower-supplies%2Fchargers%2F&h=451&w=607&tbnid=rPVvZ4fEOLl97M%3A&zoom=1&docid=d5DccEoEApTicM&ei=2VHHU8q1BIyn8AX88oHoCQ&tbm=isch&ved=0CCIQMygCMAI&iact=rc&uact=3&dur=2334&page=1&start=0&ndsp=15Links to more on that page.Forgot to mention a pulse transformer needed also.

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 17, 2014, 05:42:16 am
content: Sir NickOK, so what's the ideal reference voltage and temp to use for derating? 13.8v at 25C?For stationary batteries the float voltage at a given temp is always printed on a sticker on the battery.For car batteries etc. your guess is as good as mine.The float voltage (at a given temperature) also depends on whether the battery is a VRLA or has flooded cells. They differ slightly.For flooded cells it's about 13.80V @ 20 deg C.

author: cimmo
author_rank: Frequent Contributor
timestamp: July 17, 2014, 06:17:03 am
content: One thing about wet lead acid vehicular batteries that no one has mentioned yet is the fact you can easily get access to and test the electrolyte specific gravity (low maintenance types excluded).This is a relatively painless and accurate way of determining state of charge, just get a float hydrometer and maybe a little bit of maths for temp compensation and it will take away the guess work.Also very useful to diagnose an older, ailing battery as the different cells can easily have different charge levels.

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 17, 2014, 06:30:09 am
content: cimmoWell said. I agree. The SG is an accurate way of determining the state of charge.Even the so called "Low maintenance" batteries normally have a plastic panel under the sticker that can be levered off to reveal the individual cell filler holes. It is well worth checking the water level before tossing the battery. It's saved me a few batteries in the past.

author: SirNick
author_rank: Frequent Contributor
timestamp: July 18, 2014, 07:39:52 pm
content: Thanks for the comments everyone. And the schematics, tautech -- that page has quite a few to chew on.It has been a while since I've seen a car battery thatwasn'tmaintenance free. Had no idea the caps came offandwent back on again. Good to know.