# charging times for batteries

Options
How do you go about figuring the amount of time it takes to charge a battery?
If you have a pv at 12v and x amps and a 12v battery of say 80ah that's down to y volts, how long would the pv cell take to charge it?

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: charging times for batteries

Zippo,

Without intending to sound demeaning, battery charging is substantially more complicated than the old "pour water into a tank" analogy would suggest. Key variables include nominal battery capacity, depth of discharge, charger type, bulk charging current, battery temperature, battery technology (i.e., flooded-cell vs. VRLA) and overall battery health.

Xantrex offers the following rule of thumb in their TrueCharge battery charger manual:

Charging time = (Battery capacity in Ah) x (Battery depth of discharge in %) / (Charging current) x 80%

For example, the calculation for using a 10 A charger to charge a Group 27 size flooded-cell battery rated at 105 Ah that is 40% discharged (42 Ah removed) would look like this:

105 Ah x 40% / 10 A x 80% = 5.25 hours.

To paraphrase YMMV, your charge time may vary.

HTH,
Jim / crewzer
• Solar Expert Posts: 10,300 ✭✭✭✭
Options
Re: charging times for batteries

that last 80% may be a bit higher for some agms as this is in reference to charging efficiency. :-D the formula is basically sound for general use though.
• Solar Expert Posts: 1,484 ✭✭✭✭
Options
Re: charging times for batteries

The efficiency of charging is a function of where in the state of charge the battery is at. The 80% number might be okay as an overall average of the total recharge cycle.

The basic chemistry of the charge and discharge cycles to converter lead-sulphate to/from lead on the plates and water to/from sulfuric acid in electrolyte generates only the molecular change of the two compounds and electron movement. No heat or bubbles are part of this primary chemical reaction. So first indicator of poorer recharging efficiency is presence of bubbling, and later, battery heating.

If a battery is at 50% state of charge, the constant current pushed into the battery will generally be very efficient and can be in the high 90's % as a pure energy conversion processes. The high charging efficiency relies on good pairing availability of the lead-sulfate with water molecules in the electrolyte. The problem is the electrolyte needs a little time to diffuse the constantly changing solution mixture of water and sulfuric acid. The same localized depletion problem occurs during higher current discharge. Both the charge and discharge current rate dictates how fast the chemical conversion happens. If the electrolyte does not have enough time to physically diffuse then the plate are locally starved of water during a recharge.

There are elaborate battery systems that have an electrolyte pump to help accelerate the mixing process allowing for faster charge and discharge currents.

The plates have some series resistance which creates I^2 *R loss which is part of heat generation. As the battery gets above about 85% state of charge the concentration of available water thins out (specific density increases) and much of the plate surfaces have already been converted back to lead. It gets harder for the water molecule to find the remaining lead-sulfate molecule on the plate surface. The applied current (energy) has to go somewhere and the result is current through the already recharged lead into the electrolyte will bust water molecules into hydrogen and oxygen. This is bad from a energy conversion point of view and further reduces the available water needed for the recharge chemical reaction but has a side benefit of the gas bubbling helps to diffuse the electrolyte pushing away the higher concentration of sulfuric acid at the plate surface so water can get in and recombine with remaining lead-sulfate on the plate.

What all this means is you can push the charge current pretty hard up to the point that state of charge is about 85% then you have to back it down for remaining 15% recharge. You can have a state of charge calculator to predict where you are in state of charge but these can get pretty inaccurate after several cycles. The only direct observation you have is the battery voltage and the amount of out-gassing coming from the battery (and heat although if heat is significant ,things have been pushed too far). The battery voltage is a function of the recharge current push but generally you will find the 85/15% point at about 13.3 to 13.8 vdc given a recharge constant current of between 15% A-H to 35% A-H battery rating in recharge amps.

From an energy efficiency point of view, it takes almost as much recharge energy to accomplish the last 15% recharge as it does to accomplish 85% state of charge assuming you start from a 30% to 40% state of charge point on the battery.

If you are constantly recharging from a 70% to 80% state of charge the overall energy efficiency will be poorer, in the 20% to 30% range.

So, back to your original question on how long it takes to recharge a battery. Beside having the 30-50% A-H current available from your charger, it depends how how hard you want to stress the battery on the last 15% of capacity. The harder you push the last 15%, the more water is depleted from battery and the more the positive plate support structure gets oxidized from the oxygen bubbles.

My rule of thumb is half time spent getting to 85%, other half time spent getting from 85% to 100%. At initial stage 1 constant current of 25% to 30% AH rate, that translates to about 3 hours to get from 40% state of charge to fully recharged with 85% SOC point at about 1.5 hours into recharge cycle.

During hurricane power outages where I am recharging from gasoline generator, I terminate early, near the 85% point, which gets me the best kW-H's per gallon of gas. You can not do this forever though because batteries will eventually sulfate up.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: charging times for batteries

RC,

Nice write up on some of the intricacies of battery charging. In general, I’m in agreement with many of the concepts in your post, but I have to admit that there are a couple of details that have piqued my interest, as they vary significantly from my research, references and recommendations.

In no particular order:
If you are constantly recharging from a 70% to 80% state of charge the overall energy efficiency will be poorer, in the 20% to 30% range.

While I agree that top-of-charge recharging, generally associated with recharging’s absorption phase, is less efficient than incremental recharging at lower SOC’s (bulk recharging phase), I’ve just never seen or experienced anything the supports the “20% to 30%” numbers. Instead, Sandia Labs found incremental recharging from a ~75% SOC to be ~80% efficient.
Beside having the 30-50% A-H current available from your charger

30% to 50% of battery C (Ah) in charging current may be fine for AGM batteries during the bulk stage, but smaller values are generally recommended for flooded-cell batteries. Examples include Trojan’s recommendation of 10% to 13%, and Xantrex’ battery bank size recommendation for the 20 A version of their TC battery chargers is from 100 Ah (20%) to 400 Ah (5%). This site’s sponsor, NAWS, has recommended “1 W of solar per battery Ah”. For a nominal 12 V system that translates into ~6% charge current.

Another problem, in my view, with a charger this large is that it will operate at fairly light load in the absorption stage while the charge voltage is fixed and the charge current slowly declines. In other words, I question whether such a large charger makes sense from technical, practical or economic perspectives.

My own battery bank can be used as the basis of an example of my reservation with this particular issue. Following just the low end of the “30-50%” suggestion, an appropriate charger for my 800 Ah battery bank would be 240 A.  :-o  Not only would such a charger be expensive and difficult to find, 90% of its charging capacity would sit idle after my bank switched from bulk to absorption mode, where my bank’s typical initial absorption mode current is ~25 A.
The harder you push the last 15%, the more water is depleted from battery

Once a charger has hit the absorption voltage set point, it will go into constant voltage mode.  I’m not sure I understand the concept of “pushing harder”, which would require purposely exceeding the absorption voltage set point and defeat its purpose of effectively charging the battery and minimizing outgassing.

Can you share some of your references and insights on these issues? And, FWIW, be forewarned: I’ve been chastised in the past for venturing too deeply into topics in this “basics” section of the forum.   Nonetheless, I for one am looking forward to leaning more from your many years of practical experience in this field.

Thanks, and welcome to the forum!

Regards,
Jim / crewzer
• Solar Expert Posts: 1,484 ✭✭✭✭
Options
Re: charging times for batteries

On efficiency on last 20% of top of charge. I have to admit, part of the efficiency loss is based on my experience on recharging from a generator and the time/gasoline useage. I will say however that this may apply to some grid charger and solar chargers also so it become a judgement call on what is encompassed in the overall charging efficiency. If the charger efficiency drops off during the absorbtion phase then I include it is overall efficiency.

The maximum recharge rate is a very hotly debated topic. My view, at least during the bulk charging phase, is if you can discharge the battery at given rate then you can recharge it at that rate. A lower discharge and recharge is always going to be easier on the battery, the question is how much easier on the battery. If you push current too high (charge or discharge) you can actually eject the not too structurally sound sponge lead and associated lead-sulfate right off the plates. When this happens it (hopefully) falls to bottom of battery but you have lost some of the plate material that can never be recovered.

This current limit is not alway constant as the battery gets above 13.5 vdc where the absorption phase starts to drop current. This is where the batteries really start to outgas. There is variance on this transition period between three phase chargers. Some push bulk limited current at constant level until batteries reach bulk limit voltage (14.3 vdc or so). My Trace SW inverters start to pull back on the bulk current push about about 13.6 vdc. They gradually taper off until the Bulk limit voltage is reached then drop to float voltage. In this transition from Bulk to Float setting the SW Trace (Xantex) actual start to pull current out of the just charged battery to bring it to float voltage level. There is another problem (in my opinion) when series stacked Trace inverters are ganged charging. You never can get twice the maximum current out of a stacked setup. But this better to be a topic for a different parent post.

Consistance and uniformity is key to battery health and longevity. Higher currents, discharge or charge, will likely create current bunching at various spots on the plate, likely at the corners. Over time this creates variance in the increment square centimeter by sq. centimeter based condition of the plate and starts a downward spiral of battery health. I also view the repeated discharge/recharge process similar to a cut/healing/scaring. As more 'mutation' of the physical plates build up, the consistancy and uniformity degrade, further accelerating more deformity on successive cycling.

As to pushing harder in absorbtion phase, it is just like equalization charging. You can push a higher current into the battery in the final top off stage by raising the bulk voltage limit to 14.5v to 15 vdc. This is harder on the battery but gets most of the last 10% top-off-charge in a shorter period of time. As with equalization, there really kicks up the outgassing.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: charging times for batteries

RC,

Interesting perspectives!

I kind of guessed that you may have included something like generator and time/gasoline usage in the numbers. While they no doubt make sense from a system perspective, I’m just not used to seeing generator- and/or charger efficiency factors included in battery charging efficiency calculations (W out / W in).

I suspect that I’m in agreement with you regarding the physical toll suffered by batteries, especially flooded-cell types, as a result of high charge rates.That’s largely what was behind my question about the suggested high charge rates.

Part of the problem with high charge rates is the effect of the relatively high SG level of the thin layer of electrolyte that’s in direct contact with the plates. Conversely, the SG of the electrolyte that in contact with the plates during heavy discharge is relatively low, so it seems to me that this would be less stressful to the plates’ active material.

I find that manufacturers’ specs tend to support this assessment. For example, although Trojan recommends a charge current of just 10% to 13% of battery capacity, they also typically provide performance specs for 75 A discharge current. So, while Trojan recommends something like ~25 A (~11% of C) as a preferred charge current for their T-105 (225 Ah), they also seem to indicate that it’s OK to subject their battery to a 75 A discharge current (~33% of C). MK’s AGM battery chart includes discharge currents associated with discharging their batteries in 5 minutes! :-o I certainly don't recommend this type of abuse, BTW.

Your experience with your Traces’ charging behavior is a new one on me. My 120 VAC Statpower / Xantrex battery chargers hold their charge current steady until they hit the absorption set point, and then begin reducing current in the constant-voltage absorption stage. Assuming there’s sufficient sunlight, my Morningstar and Outback PV controller/chargers do the same.

The Xantrex TrueCharge charger can’t “pull current out” of batteries. Each of its three charging outputs is isolated with a blocking diode. Instead, the battery voltage drops as the high-SG electrolyte causing the surface charge on the battery plates dissipates (like tea steeping) into the rest of the electrolyte.

I liked your analogy of “ the repeated discharge/recharge process (being) similar to a cut/healing/scaring”. Here’s a fairly graphic example of what this might look like (page 4).

As for pushing harder in the absorption phase, I understand the electro-chemistry (mostly). I’m just not sure why anyone would do this except for periodic equalizations.

Regards,
Jim / crewzer

P.S. And that, my friends, FWIW, was post #999.
• Solar Expert Posts: 1,484 ✭✭✭✭
Options
Re: charging times for batteries

Thanks, the picture is a good example of the harm electrolyte stratification can do.

Shows the need to mix things up with some 'bubbles' periodically.
Options
Re: charging times for batteries

Great discussion. Lots of great info, experience and references. Thanks from a bystander to the conversation!

I was wondering.... Do you have any thoughts on what equalization of a flooded lead acid battery should look like from the top? I mean, one bubble a minute, or should it fizzle like an alkeseltzer? The Sandia reference makes me think I should equalize, I'm just not sure how hard. Any thoughts?
Options
Re: charging times for batteries

Greetings,
I agree with IslandBoy! This is a first class post. Thanks to all contributers for the write up.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: charging times for batteries

Island Boy,

My experience with my old flooded-cell batteries was that it was too hard to visually determine a battery cell’s bubbling / boiling / roiling during absorption phase- or EQ phase charging. The combination of the fairly small filler hole, the cell’s dark interior and my unwillingness to place an eye anywhere near splashing battery acid, even with safety glasses, made this a non-starter for me.

I relied instead on sound. A flooded-cell battery in absorption phase (~14.4 V) will emit a soft but regular simmering sound as the electrolyte gently bubbles. During EQ (~15.1 to 15.5 V), the bubbling sounds (and is) more rapid and intense.

Bill Darden and Trojan offer some good recommendations on how and when to equalize your batteries.

Thanks for the compliments, Gents.

HTH,
Jim / crewzer