Battery charger

[abel.partida]

If while charging a battery a big problem is generating heat, then a charger for several batteries in series is less efficient than a charger per battery, is this true?.

[todd.martin]

Hi Abel,

That's a great question!

For a well balanced battery pack, charging in series will not cause any particular heat issues over charging batteries separately.  For an EV, you may have 10-30 batteries in your vehicle.  It is rare for someone to charge batteries individually on a regular basis for an EV because:

1) If you charge them one at a time, this really slows down the charging process.
2) If you charge them all at once in parallel, you need a charger for each battery (that's a lot of chargers).
3) Charging batteries in parallel requires a lot more wires and connections (for a 13 battery pack, you need 13 wires instead of 1 long wire.  This is extra weight, mess, and headaches.
4) Extra care must be taken to ensure each charger is topping off its battery to the same voltage.
5) Each charger has its own plug, requiring an outlet.

As for efficiency, I think in practice that the quality of the charger is more important than how it is used.  Using a cheap 6V or 12V charger from a Walmart will not provide the same efficiency as a dedicated EV designed charger.

[abel.partida]

Since I’m an electrical eng. I find interesting this subject. I have some questions on the details you listed but first, since I don’t have an EV jet, a question on replacing the batteries. when replacing the batteries (I assume you replace them all at the same time), I am guessing at least 10%-20% of the batteries will have around 10 % of useful life left am I correct?.

[todd.martin]

Hi Abel,

People do on occasion swap out a bad battery or two.  This happens when a battery has a premature failure, due to a manufacturing defect or some misfortune.  In general, it is much preferred to swap out the entire pack at one time instead of selectively replacing the weakest battery.

The biggest reason why you want to replace the entire pack at once is to keep the batteries balanced throughout their charge and discharge cycles.  If you have 12 batteries that has a 10 amp hour capacity and 1 with a 50 amp hour capacity, your pack will still have only a 10 amp hour range.  When charging in series, the new battery will be overcharged as it waits for the old batteries to be brought up to voltage.

Batteries decline slowly in capacity, when new you get 100% and after a few years maybe 50%.  I recommend that you pick out an EV that has at least twice the range that you need.  This will allow you to drive on the same batteries longer before needing to replace them and give you buffer for cold weather, missed turns, or forgetting to plug in.

 

[abel.partida]

Hi Todd,
Trying to go deeper understanding of the above practice, lets reverse the case, you have 12 batteries that have 50 amp /hr capacity and 1 with 10 amp/hr, the 10 amp/hr battery has equivalent circuit that shows an internal resistance much higher but the remaining batteries will force (still high) current through the 10 amp/hr battery, the power dissipation (heat due to the internal resistance) on this battery will be much higher, and internal heat is not good for the battery.
Even in the charging cycle same situation is present, does any battery manufacturer show performance curves on charging several batteries in series?

In electronics when trying to increase the voltage (e.g. two power supplies in series) rule #1 is that the current from the first power supply should not go through the second and current from the second should not go through the first, this is corrected by simply adding a diode at the output of each power supply, then any excess of current at each power supply will be diverted to through the diode, I don’t understand why not apply the same thinking in batteries     

[todd.martin]

Hi Abel,

A diode placed between each battery would be impractical.  A diode allows current to flow in one direction, but not in the reverse direction.  Since batteries in an EV experience charging (one direction) and discharging (reverse direction), diodes would prevent the EV from doing one of the two required operations.

Many well-designed EVs do have devices placed between each battery to regulate the current flow.  Some use a BattEQ or Powercheq device.  Each of these monitors the voltage between adjacent batteries, and actively draws power from the stronger battery and feeds it to the weaker.

Other designs use voltage regulators (such as a Manzanita Mk.3 Regulator).  These keep an individual battey in a series string from having their voltage go too high.  I believe that they divert the excess current into a resistor.

In my case, the Force has neither, relying on the uniformity of the Deka Gel batteries to stay in balance.

   

[ted.lowe]

second should not go through the first, this is corrected by simply adding a diode at the output of each power supply, then any excess of current at each power supply will be diverted to through the diode, I don’t understand why not apply the same thinking in batteries     

As Todd pointed out, i don't think the diode idea wouldn't be all that simple (wish it was!).  The regulators he mentions are just about essential for series connected sealed lead acid batteries (AGM, Gel) because they are notorious for variances in charging/discharging over time.  We've had 3 or more members in recent times kill their sealed batteries prematurely due to uneven charging.  Flooded lead acid batteries due introduce some routine maintenance (periodically filling with distilled water), but you can equalize them frequently to keep all their voltages very close to each other.

Another idea that is coming into vogue in the last couple years are round-robin chargers.  These units have a charge/sense lead on each battery and provide a timed constant current to each individual battery on a round robin basis.  Once all the batteries get close to being full, some are skipped (by the logic in the microcontroller) if they are fuller than their neighbors.   This is so clever and so efficient too, and will result in longer lasting series battery packs.  Longtime member Ed Meyer is working on such a design and i hope eventually FVEAA members will have access to his design at a very reasonable price.   Then... say good by to series charging forever!

Kind regards,
ted

[ted.sanders]

Ted,

Would this approach be useable in charging NiMH and Lithium batteries.  As I understand these packs use both parallel and series arrangements.  Either cells connected in paralled to provide the desires AHs then in series to provide the desired voltage.  Or a group of series batteries of the proper voltage are connected in parallel to give the desired AHs.  Is this correct?

[ted.lowe]

Would this approach be useable in charging NiMH and Lithium batteries.  As I understand these packs use both parallel and series arrangements.  Either cells connected in paralled to provide the desires AHs then in series to provide the desired voltage.  Or a group of series batteries of the proper voltage are connected in parallel to give the desired AHs.  Is this correct?

Hey Ted,
i guess it depends more on battery configuration (series, series/parallel or parallel/series) more than the battery chemistry.   The Ed Meyer round-robin concept can be parameterized for a specific battery chemistry and battery model.  He recommended contacting the manufacturer for the exact details of the BEST way to charge a battery, constant amperage, voltage, time segments, etc.   The plan is to have a programmable microcontroller in the 'new Ed Meyer' charger that will support such parameterization.

As far as non-series only configurations, i'm not sure about the round-robin approach.  For example, a common configuration with the Optima Yellow Tops is to have have either a series string of buddy-pairs or two parallel strings.   This configuration is used with YT's because they are smaller than needed for acceptable range.
But as soon as you parallel strings or buddy-pairs, then charging one battery will affect the others in the circuit.  Perhaps one could design in a method to break the circuit between the parallel batteries for the purpose of charging.

i think newer Lithium battery packs take a whole different approach at charging, BMS (battery managements systems) which basically has one charger circuit per battery!  That's partly why they are so EXPENSIVE!

Kind regards,
ted

[abel.partida]

Hi Todd
Let me try to understand a point you made “A diode placed between each battery would be impractical.  A diode allows current to flow in one direction, but not in the reverse direction.  Since batteries in an EV experience charging (one direction) and discharging (reverse direction), diodes would prevent the EV from doing one of the two required operations.”

When I said to add a diode, this diode is in parallel to the battery
(each) did you picture it this way?
I don’t see how can it prevent either task.

[todd.martin]

Hi Abel,

Thank you for clarifying this.  I thought you meant between each battery, instead of parallel to them.  Since I am not an electrical expert, I can't tell you what the results would be.

Best regards,

Todd Martin

[abel.partida]

Hi Todd
I’ll like to try to agree on the issue of one charger vs. several chargers, I believe that it is a better option to have one charger per battery, could you check the following link and tell me your conclusion?

http://visforvoltage.org/forum/batteries-and-chargers/1561

Thanks.
Abel Partida

[todd.martin]

Hi Abel,

Thanks for the link; I'll try to recap.

1) For Flooded lead-acid batteries, a single charger may be used without balancing by using an equalizing charge.  While properly performed equalization charges will slightly reduce the cycle life of an individual battery, it is a cheap and effective way to keep floodeds working properly in an EV.  A balancer can extend the cycle life and effective range of floodeds.
2) For AGM and most Gel batteries, a single charger may be used, but will need a balancer.  In smaller strings with very well made Gel batteries (such as in my Force), you can get away with omitting the balancer (although a balancer would still help extend cycle life and effective range).
3) For Lithium Ion batteries, a balancing system is required.

The article you mentioned recommends individual chargers for Ebikes and Escooters as a more economical choice than a balancer, which is true. 

However, using individual chargers instead of a balancer is not as good as a balancer. 

BattEQ & PowerCheq balance during discharge and while idle.  Balancing during discharge helps extend effective range and reduces the  depth of discharge percentage used on the weakest battery for any given trip.

One important point is that individual 6V or 12V chargers cannot be hooked directly up to a 144V or 156V battery pack.  You have to first disconnect the batteries from the string to use your 12 to 26 chargers that you have in your garage.  For convenience, and for many of the reasons outlined earlier, a single charger is best in most cases.   

Todd Martin