Battery charging

[WotEver]

On 04/09/2020 at 07:03, TheBiscuits said:

Request a mod edit by reporting your original post and including the text you want adding.  It's a good bit to add to the topic.

Well I did so on Friday after spending a fair bit of time wording the para. No response so far...

[Richard10002]

In terms of time to go before 100% SOC, (tail current approx 1% of capacity), I used a kind of half life estimation:

 

With an 80A charger and a 450Ah bank, once the bank is in absorption mode, (i.e. drawing less than 80A) - If the bank is drawing 79A, it will do this for 1 hour, then it will draw 40A for an hour, then 20A for an hour, then 10A for an hour, then 5A for an hour, and that was pretty close to my tail current.

 

It wasn't perfect, but gave a reasonable estimation, which got more accurate as the amps drawn fell.

 

I knew with reasonable accuracy that the last 20Ah to get to 100% SOC would take 3 hours or so.

 

(With Lithium batteries, if I want to put 180Ah into the bank, I charge for 3 hours with my 60A charger, and don't need to worry if they are full or not - better if they are not.)

[Keeping Up]

19 minutes ago, Richard10002 said:

In terms of time to go before 100% SOC, (tail current approx 1% of capacity), I used a kind of half life estimation:

 

With an 80A charger and a 450Ah bank, once the bank is in absorption mode, (i.e. drawing less than 80A) - If the bank is drawing 79A, it will do this for 1 hour, then it will draw 40A for an hour, then 20A for an hour, then 10A for an hour, then 5A for an hour, and that was pretty close to my tail current.

 

It wasn't perfect, but gave a reasonable estimation, which got more accurate as the amps drawn fell.

 

I knew with reasonable accuracy that the last 20Ah to get to 100% SOC would take 3 hours or so.

 

(With Lithium batteries, if I want to put 180Ah into the bank, I charge for 3 hours with my 60A charger, and don't need to worry if they are full or not - better if they are not.)

Or to put it another way - no longer how long you charge your batteries for, you are always exactly one hour away from charging them fully.

[WotEver]

14 minutes ago, Keeping Up said:

Or to put it another way - no longer how long you charge your batteries for, you are always exactly one hour away from charging them fully.

Lol. So true

[Ronaldo47]

The reference in the battery university site about sulphation occurring if the voltage falls below 2.05V per cell is a bit misleading. Sulphation occurs as part of the normal discharge cycle. But there is sulphation and sulphation. In a fully charged battery, the positive plates have a layer of chocolate brown lead peroxide, while the negative plates have a coating of grey spongy lead. During discharge, these progressively react with the sulphuric acid to produce areas of lead sulphate on both plates, initially at the surface and progressively deeper as discharge proceeds. The production of this type of non-crystalline lead sulphate is readily reversed on charging to regenerate the original lead peroxide and spongy lead, but only if the voltage never falls below 1.8V per cell (10.8V for a 12V battery with all cells in good condition). At 1.75V per cell, the lead sulphate becomes converted to a hard crystalline form that is dificult  or impossible to be changed back to lead peroxide and spongy lead.  The normal sulphate has a higher electrical resistance than the original coatings, so if an excessively high current is used to charge a deeply discharged battery, there is a risk of localised heating where the current is confined to the regions between the sulphate,  leading to differential thermal expansion between the different plate materials, and consequential shedding of active material and thus loss of capacity. The old way of checking if a battery was fully charged, involved measuring the specific gravity of the acid and visually inspecting the plates, but this is not normally practical these days, especially on a canal.boat. 

Information from a 1940's book that deals solely with accumulator charging. Admittedly an old book, but modern lead acid batteries must surely operate with the same fundamental chemistry and electro-chemical reactions. 

 

Edited September 7, 2020 by Ronaldo47
Typos

[WotEver]

21 minutes ago, Ronaldo47 said:

The production of this type of non-crystalline lead sulphate is readily reversed on charging to regenerate the original lead peroxide and spongy lead...

A very misleading statement. All of the time that a lead acid battery is sat at less than 100% SoC it is growing lead sulphate crystals. The longer it remains below 100% the larger and harder those crystals become. Subsequently charging the battery most certainly does not ‘readily reverse’ that sulphation. In fact, the last little bit of sulphate can be very stubborn indeed and extremely difficult to remove. Battery University is therefore correct in its statement. A 12V LA battery does not need to be as low as 10.5V to create hardened lead sulphate, it will do it at any voltage below about 12.4V if given sufficient time. 

[Ronaldo47]

It depends how you do the charging. Some years ago when I was with Marconi I salvaged an apparently completely dead  (0V) Sonnenschein 12V SLA battery from an old Inmarsat  "suitcase" base station that Marconi Marine had left behind as scrap when they moved to another site. Before turning to management, our office manager had been a research chemist doing battery research and development at the old GEC Hirst research labs at Wembley, and said that, if it had died due to self discharge,  it might respond to being connected to a voltage source via a high resistance. I used a 1960's 12V trickle charger that provides about 15V  off- load, and a 12V 2.2W MES bulb for the current-limiting resistance. Nothing happened for several days, but eventually the bulb started to glow dimly, and got progressively brighter with time. It took a couple of weeks, but the battery was eventually restored to about 50% of its rated capacity and I was able to use it to power an electric tyre pump for my bicycle for several years before it finally died (with periodic recharging of course). 

 

You cannot use this technique with modern smart battery chargers, as they rely on detecting a voltage in the range expected for the battery to be charged. If no volts are detected,  no volts will be output as it thinks no battery has been connected. It can be a problem with very sophisticated chargers designed to automatically detect and charge 6V, 12V and 24V batteries. If you have let your 12V car battery get really flat by leaving the lights on, its terminal voltage can be in the range that the charger expects to see from a 6V battery, so it will only attempt to charge it to 6V.  

 

I guess that, commercially, it would cost more than a battery is worth to attempt resoration of dead batteries, more cost-effective to scrap and buy new replacements, time is money.  An engineer at Plessey who had been in the Royal Signals regiment said that they used to do a discharge-charge cycle on their batteries every 6 months or so to keep them in good condition, first fully discharging to a safe voltage, then recharging until all the cells gassed. The object was to  reform the active material of the plates, a technique described and recommended in my old accumulator charging book. It is true that  SLA batteries were not around then, and manufacturers' current instructions should be followed. Nonethless, apparently dead ones can be partially resurrected.

 

Edited September 8, 2020 by Ronaldo47
Typo, further thoughts

[WotEver]

The post above reinforces my point. The flat Marconi battery, despite careful and time consuming charging never recovered beyond 50% due, partially, to hardened lead sulphate that could not be converted back. I’m sure it also suffered from some plate erosion too. 

[David Mack]

On 06/09/2020 at 00:00, WotEver said:

 

A decent Solar installation will result in a much longer life for your batteries. You can start the charge off with a bit of engine running then leave the solar to take care of the long, slow, boring bit. But it’s not a lot of use from November to March. 

 

Probably true if you are using power all year (e.g. liveaboard boater). But I find that as a non liveaboard, solar has kept my batteries topped up through the winter. You don't need a lot of sunlight to make up for the self discharge of batteries that are not supplying power.

[WotEver]

8 hours ago, David Mack said:

 

Probably true if you are using power all year (e.g. liveaboard boater). But I find that as a non liveaboard, solar has kept my batteries topped up through the winter. You don't need a lot of sunlight to make up for the self discharge of batteries that are not supplying power.

All very true. 

[Keeping Up]

10 hours ago, David Mack said:

 

Probably true if you are using power all year (e.g. liveaboard boater). But I find that as a non liveaboard, solar has kept my batteries topped up through the winter. You don't need a lot of sunlight to make up for the self discharge of batteries that are not supplying power.

Better than that, while we were away from the boat from February until July, our small (110 watt) panels put nearly 300 Ah extra into our already-charged 3-year-old batteries and rejuvenated them from "ready for replacement" up to over 80% of their original capacity.