Testing the voltage of a battery being charged

[Sunking]

This has me thinking about my LiFePo4 battery pack on my drag bike.

Dude racing is another story. For racing you want the best batteries money can buy with the highest discharge C rate you can get. You would also want to cram every AH possible into the battery which will shorten battery life. Money is no object with racing, you buy the best money can buy. I would also assume the battery pack is replaced frequently as racing conditions would put a heavy toll on the batteries and capacity would degrade after 10 to 20 races.

[thastinger]

Something is wrong here. For a Lipo 4.2 vpc (3S = 12.6). 4.1 or 12.3 for 3S is roughly 85 to 90% which is fine if you are comfortable with it.


I keep mine in a Zip Lock bag with a silica pack to keep moisture low. Don't freeze them, 35 to 40 is perfect for storage. Just be sure to warm them up a bit, then charge them and go fly.

Well yes and no. A good Float Charger is strictly CV with a current limit. Float Chargers are excellent and a gentle charge, but not fast. Float Chargers are primarily used in Emergency Stand By power systems like Telephone Office DC Power Plants and UPS. They are also an excellent choice for storing batteries long term.

2, 3, and 4 stage chargers are both CC and CV. CC mode is used for Bulk only to get you to 90% charged as fast as possible. Absorb, Float, and EQ are CV mode with current limit. For solar chargers most good ones have either 3 (Bulk, Absorb, and Float), and 4 stage adds EQ. With that said with solar set Bulk = Absorb to get fully charged ASAP.

This has me thinking about my LiFePo4 battery pack on my drag bike. I have 2 of them 4S4P configuration, one I built and one I bought. The one I built does not have the balance circuit but the one I bought does and I have a fancy charger to go with it. I use the one I built as a backup and the one I bought stays in the bike. The bike is a total loss system so I charge the battery with a B&D charger set on 10A at the races and use the balance charger just a couple of times a year for equalizing the cells but they are never far enough out to be concerned. Have done this about 3 years now without issue but I need to check my balance charger because I remember having to dig down a few menu screens to find LiFe settings, it has the ability to build custom charge settings so maybe I need to do that and save it.

[SunEagle]

Something is wrong here. For a Lipo 4.2 vpc (3S = 12.6). 4.1 or 12.3 for 3S is roughly 85 to 90% which is fine if you are comfortable with it.

I went back and checked all of my charge records and the majority of them had the same 12.3v end results. There was one time that I charged one in the field on the "fast" charge setup and that one got my battery to 12.6v. I didn't pay attention to it before but I will research to see if the "slow" charge setup has limited the voltage for some reason. Maybe to extend the battery life.

Well yes and no. A good Float Charger is strictly CV with a current limit. Float Chargers are excellent and a gentle charge, but not fast. Float Chargers are primarily used in Emergency Stand By power systems like Telephone Office DC Power Plants and UPS. They are also an excellent choice for storing batteries long term.

2, 3, and 4 stage chargers are both CC and CV. CC mode is used for Bulk only to get you to 90% charged as fast as possible. Absorb, Float, and EQ are CV mode with current limit. For solar chargers most good ones have either 3 (Bulk, Absorb, and Float), and 4 stage adds EQ. With that said with solar set Bulk = Absorb to get fully charged ASAP.

That makes sense. A more sophisticated charger should have more functionality. Thanks for the clarification.

[Sunking]

The maximum current is regulated by the solar panel. The panel power, voltage, and current is matched up to the battery.

[spectralcanine]

Thanks!

I suddenly thought about looking at the datasheet yesterday, and saw that the battery should indeed get ~2.45V per cell (14.7V), so it seems the internet mislead me on this part.
I ordered a DC-DC converter to regulate the voltage, but I am not sure how to limit the current.
The datasheet says the battery should not get over 2A "initially", what does initially mean in this context?
The DC-DC regulator also doesn't like anything above 2A.
How does one go on about changing currents? my knowledge in electronics is pretty basic, and I can't seem to google any simple ways to limit the current up to 2A.
If I have good sun and get the full 3.6A the cells are supposed to output, it would probably fry the regulator.

On the consumption part, I have two sets of 3 1W power leds connected in series to two drivers that feed them ~300mA, so I don't think it's a very big issue for the battery.

The Arduino can't work with voltages above 5V, so I can't control the voltage with it indeed. In fact to be able to even read the voltage I need a voltage divider of 1/3 or more.

And I must say that no matter how much I read about C rates, I still have no idea what they are.

[PNjunction]

I made a solar panel from 36 cells connected in series, each with specs of 0.5V and 3.6A.
At optimal conditions it should give 18V, and in open circuit checks it seems to give between 20V when it's not hot, and gets as low as 14-15V when it warms up too much (any good ways to cool it?).

Sounds about right for a DIY panel. Note that the loose-cells you get are NOT grade A, but factory rejects so they may have accellerated heating issues dropping you down to the 14v range. A panel that exhibits 20v when cool, and only gets to about 160F in the sun may not drop as much - maybe down to 17v OCV. But so far ok for DIY.

I am charging a Lead Acid AGM 12V 5Ah battery with it.

Cool. That means for these typical lead-calcium ups-type agm's, you don't exceed 14.7v charge, and have a typical maximum inrush current of about 0.25 to 0.3C. That means you should not be supplying it with anything larger than 1.25a to 1.5a. Sometimes that is printed on the case, but reputable manufacturers like PowerSonic, B&B etc have online docs and charts. Your diy panel is supplying about 2 times the recommended max for a 5ah agm.

When it is disconnected for some time (hours?), it balances at voltages that make sense, like 13.x after a charge. However, when the panel is connected, the voltage spikes up to 15-16V.

That means you don't have any sort of absorb voltage limiting - in the case of these agm's, in a cyclic application, no more than about 14.6 to 14.8v should ever be applied. If you do more, you are corroding the plates with high voltage, basically doing an EQ on them which most manufacturers say not to do. Look for venting, hissing, buckling plates. A little handheld IR temp gun can help you keep an eye on it.

You'll also need current limiting, and perhaps the easiest is to just make sure your system is not capable of more than about 1.5a. Since that 5ah battery is now probably nicely toasted from your high-voltage, I'd upgrade to a 10ah agm, and program that arduino to never go above 14.7 volts.

And on the opposite side, when I put some load on the battery, in the form of power leds that it's supposed to run on a daily basis, the voltage spikes down.

Quite natural. If the leds draw no more than C/20 to C/10, (250 to 500ma total), then you are not heavily exceeding Peukert, and the voltages below will be in the ballpark even under load. Ideally, these voltages should be measured after a period of at least 4 hours of no charge and no load:

12.8 + 100% SOC
12.5 75% SOC
12.2 50% SOC
12.0 25% SOC

Your battery manufacturer will have more specific voltages and may include handy discharge/current charts to help calibrate the actual SOC under a wider variety of loads. Note that most people avoid drawing any larger than 0.25C otherwise your battery will only last minutes instead of hours for example.

How then do I check the real voltage for the charger, so I can know when to stop charging to avoid over charging, and whether I have to disconnect the power leds because the battery is too empty, to avoid over discharge?

Let the battery do the regulating - all you have to do is provide a maximum absorb voltage and limit the maximum inrush current. It is the typical 2 or 3 stage charger.
With your charger set to run at no more than 14.7 volts, the battery will eat up as much current as it can in the bulk stage. Your panel is capable of too much for these batteries, so either cut your panel output in half, or even easier, use a 10ah agm!

Once the battery voltage reaches 14.7 volts, and with your arduino charger limited to not ever go higher than this, the *BATTERY* will naturally self regulate itself by absorbing what it can and current falls naturally as dictated by the battery. Kind of like you eating a desert after a heavy meal. You are already full, but perhaps you can absorb just a little more pumpkin pie.

When to stop absorbing? If you have a way to measure current, then you can do either:
Charge until you reach C/100, and then float at 13.5 volts ....or
Charge until you reach C/1000. Float optional.

Better yet, since all batteries age from natural causes, trying to reach a specific current may not ever get there due to electrolyte dry-out. In this case, program your system to detect no major changes in current for 3 hours, and then drop to float. If no current sensing is available, then perhaps you could just incorporate a timer, say limiting the absorb time from when the Arduino detects 14.7v to no more than 4 hours when a forced change to float 13.5 should occur. Having both timers and current sensing is ideal.

This is basically what most 2 or 3 stage "smart" chargers do. From your description, it sounds like your Arduino is not capable of maintaining a voltage level, but only turning a circuit on and off at preset voltage levels. In this case, the only thing you can do is a 1970's style hysteresis "ping pong", where you turn the panel on when the battery voltage reaches 13.2v at the low setting, and turn off at the 14.7v setting. I don't advise it though for battery longevity.

You could do the old school fake-pwm by putting your low and high voltages very close together, ie 14.2v for low and 14.7 high. Still, read the Morningstar archival docs on why they went to a true pwm duty cycle and not a tight-hysteresis model.

At this level, if you really want to have some fun, and not worry too much about current limiting a 5ah agm, then look into Hawker Cyclon monoblocs. Pure-lead agm. Takes massive current input (within reason), but still DONT exceed 15 volts. They look like baby JCI/Optima's, but are really Enersys/Odyssey/Hawker family. Thing is, these babies like to see about 0.4C minimum under cyclic ops, so you'd want to supply a minimum of 2a charge from your solar/charge-controller system.

[Sunking]

My Revolectrix battery charger has a number of different ways to charge LiPo's. I usually use the "slow" charge rate setup which takes about an hour to go from about 11.5 volts to 12.3 v which it determines to be 100% charged.

Something is wrong here. For a Lipo 4.2 vpc (3S = 12.6). 4.1 or 12.3 for 3S is roughly 85 to 90% which is fine if you are comfortable with it.

While I keep my batteries in a safe cool place I don't usually leave them in the frig door so I better think about doing so in the future.

I keep mine in a Zip Lock bag with a silica pack to keep moisture low. Don't freeze them, 35 to 40 is perfect for storage. Just be sure to warm them up a bit, then charge them and go fly.

As for FLA battery chargers, do they all have both the CV & CC type charging?

Well yes and no. A good Float Charger is strictly CV with a current limit. Float Chargers are excellent and a gentle charge, but not fast. Float Chargers are primarily used in Emergency Stand By power systems like Telephone Office DC Power Plants and UPS. They are also an excellent choice for storing batteries long term.

2, 3, and 4 stage chargers are both CC and CV. CC mode is used for Bulk only to get you to 90% charged as fast as possible. Absorb, Float, and EQ are CV mode with current limit. For solar chargers most good ones have either 3 (Bulk, Absorb, and Float), and 4 stage adds EQ. With that said with solar set Bulk = Absorb to get fully charged ASAP.

[SunEagle]

Thanks Sunking. That was a great lesson in battery charging. I guess I knew about current limiting but didn't put the pieces together.

My Revolectrix battery charger has a number of different ways to charge LiPo's. I usually use the "slow" charge rate setup which takes about an hour to go from about 11.5 volts to 12.3 v which it determines to be 100% charged. I have had good luck with my Turnigy's using that setup and I have yet to discharge them below 11.1 volts.

I am also familiar with the long storage voltage around 3.8v per cell and not to go below 3.3v otherwise I will shorten its life.

While I keep my batteries in a safe cool place I don't usually leave them in the frig door so I better think about doing so in the future. Thanks for that tip

As for FLA battery chargers, do they all have both the CV & CC type charging?

[Sunking]

Oh I like Shrimp. I like Prawns even better or Rock Lobster.

OK SunEagle I hope I can explain this in a way that well connect the dots for you. I know you have the background. There are two major charging algorithms used to charge all batteries. The difference between the battery chemistries is how we terminate the charge process, or know when the battery is full.

First up is Constant Current (CC). Just as the name implies we supply a specific fixed amount of current up until some set point is reached. That set point for Lead Acid is the end of the Bulk Charge Voltage. The C rate for lead acid needs to be somewhat slow not to exceed C/8 for FLA. AGM can be higher because AGM internal resistance is lower. For a 12 volt Lead Acid battery 14 to 14.4 volts. At the end of the Bulk charge the battery is at roughly 90% charged up. With FLA we also want to observe a minimum C rate of about C/12 to prevent stratification. With AGM and Gel we can go as low as C/20 to C/30 because stratification is not a problem with AGM and Gel

Lithium we also use a CC algorithm. For this explanation say a C rate of 1, some up as much as 5C. Lithium can be charge at much higher rates FLA because the charge efficiency is very close to 100%, so no heating occurs until we either charge at too high of a voltage or too much current (over charge). The CC rate terminates when the battery voltage reaches 100% State of Charge voltage. For LiPo 4.2 volts per cell (vpc) or for a 3S cell 12.6 volts. At that point the Lipo is also at about 90% fully charged up. Then we switch to Constant Voltage.

OK Constant Voltage (CV) as the name implies is a fixed precise voltage. It also has another component of being current limited to some value. CV has a few different names like ABSORB, FLOAT, and EQUALIZE. All of those are a CV algorithm. As I stated it also has a current limit aspect to it. If the current was not limited and sufficient enough you blow the battery up as soon as you connect it.

Even if it is current limited a fire or explosion could still occur if not matched correctly. For example let’s say you bring your 24 volt 100 AH battery to a cell site and want to charge it up using the 24 volt DC power plant rectifiers. The plant rectifiers are FLOAT CHARGERS current limited to 1200 amps. Your battery would blow up in your face as soon as you connect it. The cell site batteries are AGM 1600 AH batteries capable of 2C charge rate (3200 amps) so 1200 amps is well below 2C. Your 100 AH battery would receive a 12C charge rate, or roughly 100 times to much current. Boom!

Now here is where is gets interesting and you should start connecting the dots. A CV algorithm can also be used as a CC algorithm via the current limit. Take your same 24 volt 100 AH battery that is discharged; connect it to a 24 volt 15 amp Float Charger set to 27 volts and you have a perfect match. When you initially connect the battery with a terminal voltage of say 23 volts, the current limiter limits current to 15 amps and the voltage folds back to match the battery impedance. As the battery charges if we monitor the voltage is slowly rises from 23 volts at a steady 15 amps until we reach 27 volts. At that point the battery voltage is approaching the chargers Float Voltage of 27 volts. As it gets close the charger voltage does not rise above 27 volts but the Current Tapers off until the battery reaches full charge. At that point the current will be very low and will now be a Trickle Charger and only supplying the self-discharge current of a Lead Acid Battery. Understand?

OK one more step further to separate the batteries from one another. In this case LiPo. All a LiPo charger needs to be is a float charger with current limit, and a circuit to sense the charge current. That circuit needs to monitor the charge current until it tapers down to about 5 to 10% of the battery AH capacity. For example if the battery is a 3S 1000 mah battery, we apply a 12.6 volt Float Charger with a current limit of 1C or 1 amp until the current Tapers down to 5 to 10 milli-amps, then we terminate the charge by removing it immediately. At that point the battery is fully saturated and 100% charged.

Lithium batteries cannot be left on a float charger like a lead acid battery can because the chemistry is different and almost no leakage current with a Lithium battery.

Now SunEagle here are a few tips for charging your LiPo’s:

• Charge at .7C of manufactures maximum recommended charge rate. This will improve cycle life. Takes a bit longer but worth it. At the feild go ahead and charge fast between flights if you want.
• If you charger will allow you set the voltage down from 4.2 vpc to 4.15 vpc. Again some more cycle life gain but still yield 90% capacity. Some like to use 4.1 or 80% capacity but no big gain of cycle life. The big gain is down from 100 to 90. You will get a couple more years out of them.
• You know this one I think. Store them at 3.85 vpc and store in your refrigerator door or other warm spot in the fridge.
• Never ever fully discharge a LiPo. No lower than 3 volts loaded or 3.2 vpc open circuit.

Hope that helps?

[SunEagle]

Throw one on the grill for me. Love Grouper, Mahi-Mahi, and Stone Crab. You are on the right track.

Grouper it is. Also have a shrimp for that bar-bi.

[Sunking]

Crap. I'm fishing now.

Throw one on the grill for me. Love Grouper, Mahi-Mahi, and Stone Crab. You are on the right track.

[SunEagle]

No that is not what I am fishing for. Forget the balance charge for a minute. What you are describing in Top End Balance. Does you rcharger have 1 or 2 plugs to charge a Lip0. One for balance, and the other say EC3 for the main power?

What does the charge current and voltage do when you charge a lipo. Same thing happens for a Lead Acid. Difference is we terminate the as soon as something happens. When that same thing happens with a FLA we lower the voltage to float and leave it there forever if we want.

Trying to get you to think and answer your own question. I promise you know it, you just have not made the connection yet.

Hmmm. Not thinking clearly after the second beer. Gotta go read up on battery science.

Well the battery has a fixed resistance so if a specific current is reached then that will equate to a specific voltage!!! Crap. I'm fishing now.

[Sunking]

Yeah on a balance charger it puts a load on the cell with the highest voltage to partially drain it. That allows the lowest cell to catch up. I wasn't sure if that technology was used on a FLA battery.

No that is not what I am fishing for. Forget the balance charge for a minute. What you are describing in Top End Balance. Does you rcharger have 1 or 2 plugs to charge a Lip0. One for balance, and the other say EC3 for the main power?

What does the charge current and voltage do when you charge a lipo. Same thing happens for a Lead Acid. Difference is we terminate the as soon as something happens. When that same thing happens with a FLA we lower the voltage to float and leave it there forever if we want.

Trying to get you to think and answer your own question. I promise you know it, you just have not made the connection yet.

[SunEagle]

Give you a hint. Watch you LiPo charger and pay attention to the current when the battery reaches full charge. What happens?

Yeah on a balance charger it puts a load on the cell with the highest voltage to partially drain it. That allows the lowest cell to catch up. I wasn't sure if that technology was used on a FLA battery.

[Sunking]

There must be some type of circuitry that the charger uses to determine the current voltage of the battery otherwise how does it know to go from full charge status to float?

Give you a hint. Watch you LiPo charger and pay attention to the current when the battery reaches full charge. What happens?