Testing the voltage of a battery being charged

[spectralcanine]

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?).

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

In addition, I am making a charge controller for it with an Arduino.

The issue is that I have no idea how to get the correct voltage of the battery. 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.
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.

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?

I searched on Google for quite some time, but nobody mentions this for some reason.

Thanks.

[SunEagle]

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?).

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

In addition, I am making a charge controller for it with an Arduino.

The issue is that I have no idea how to get the correct voltage of the battery. 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.
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.

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?

I searched on Google for quite some time, but nobody mentions this for some reason.

Thanks.

I don't feel that charging the battery directly from the panel is good thing. Since the panel output will vary depending on the amount of sunlight so will the charging ability. It can go to low or too high. It is best to have a charge controller between the panel and battery so the battery gets a consistent charge voltage and will not get over charged.

A good charger will provide battery voltage so you won't have to guess what is going on.

[spectralcanine]

I don't feel that charging the battery directly from the panel is good thing. Since the panel output will vary depending on the amount of sunlight so will the charging ability. It can go to low or too high. It is best to have a charge controller between the panel and battery so the battery gets a consistent charge voltage and will not get over charged.

A good charger will provide battery voltage so you won't have to guess what is going on.

The whole idea is that I want to make the controller myself, but I can't get isolated readings of voltages when everything is hooked up.
Searching online shows that ~18V is what you pretty much need to charge a 12V lead acid battery, and when there isn't enough light, nothing happens (there's a diode so the battery doesn't feed the cells), so I am not too concerned on that part.
It's the voltage of the battery itself = how much it's charged, that I can't get, because it is biased by what's connected to the battery.

[SunEagle]

The whole idea is that I want to make the controller myself, but I can't get isolated readings of voltages when everything is hooked up.
Searching online shows that ~18V is what you pretty much need to charge a 12V lead acid battery, and when there isn't enough light, nothing happens (there's a diode so the battery doesn't feed the cells), so I am not too concerned on that part.
It's the voltage of the battery itself = how much it's charged, that I can't get, because it is biased by what's connected to the battery.

I would presume that the electronics used in a normal battery charger measures the voltage correctly. Otherwise it would over or under charge the battery.

You maybe able to find out more if you can get the schematics for a 3 stage charger and apply that knowledge toward your home built charger.

[Sunking]

Voltages while charging is meaningless. You can have a completely discharged battery or even a dead one with 14.4 volts indicating it is fully charged when in fact it is not.

[SunEagle]

Voltages while charging is meaningless. You can have a completely discharged battery or even a dead one with 14.4 volts indicating it is fully charged when in fact it is not.

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?

[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?

[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]

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]

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]

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]

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]

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]

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]

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.