Initial charge current limit in lead-acid batteries

Discussion in 'The Projects Forum' started by kender, Nov 17, 2011.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    0
    Colleagues,

    I’m reading-up on rules and customs for charging lead-acid batteries. It appears that they have 2 charging regimes with different charging voltages: 2.30V to 2.35 V/cell and 2.40V to 2.45 V/cell (source: Table 4-5 here). That generally agrees with charging voltages imprinted on some of the batteries. For example:
    [​IMG]
    Notice that initial charging current for Standby Use (lower charging voltage) has no limit. Charging current for Cycling Use (higher charging voltage) has a limit at 0.3C Amps. Without the limit the battery would pull 2C amps at that charging voltage.

    Here’s what I’m struggling with:
    In the vehicles, the starting battery, the system load, alternator w/ regulator are all wired in parallel. AFAIK, there is no current limiting between the alternator and the battery.
    What’s the difference between these 2 charging regimes such that charging at lower voltage doesn’t have a current limit (or doesn’t need external current limiting)?

    Any suggestion, insight or reference is really appreciated!

    Cheers,
    - Nick
     
    Last edited: Nov 18, 2011
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    That's because a normal (healthy)"12V" lead acid battery float charged at that voltage level will draw VERY little current, that's why it can be float charged indefinitely that way with very little water loss over time.

    It might pull 2c briefly if it was SEVERELY discharged, but that's not normal usage for a lead acid cell. Even when using the 14.4V charge voltage (as Chrysler autos did for many decades) the battery comes back up to that voltage very rapidly after a significant discharge (like starter crank).

    Lead acid batteries have the characteristic that their voltage comes up quickly and then the current tapers off. They are extremely tolerant of charging systems built around the voltage levels shown on the label.

    FYI: I use a standby voltage of 13.2V on my float charger but it also has temp compensation.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Here is the curve explaining it. It shows how the voltage can vary a lot but the charge level is fairly constant.
     
  4. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15
    Kender,

    NOTE The to be safe always use the batteries manufactures suggested charging schedule on all batteries.

    It took me time to wrap my head around it also, ~13.8 volts is the key, thats the voltage where the battery starts to gas and create heat. A good rule of thumb is you can charge a lead acid battery at any current you want until the battery reaches 13.8 volts, then you charge the battery using voltage and let the current tapper.

    IEI charging is a very popular charging method for lead acid batteries. What you want to avoid is heat and gassing. I found through experimenting that a battery will take in almost as much current as it can put out, so if you have a low internal resistance battery high current is appropriate.

    I - constant current until it reaches 13.8 volts (2.3 volts per cell)
    E - charge at 14.8 volts until current stops dropping (2.466 volts per cell)
    I - constant current of 2 amps until voltage stops rising (for automotive size battery, this is used to balance the cells)

    Sealed spiral wound batteries have a problem with the acid moving to the positive post post, what happens is with a hot battery the positive post will cool off first using the battery cable as a heat wick thus the liquid slowly moves in that direction. The negative post of the battery is the hotter of the two because electrons have mass and the highest point of resistance is where the electron tries to get back into the battery.
     
  5. kender

    Thread Starter Senior Member

    Jan 17, 2007
    263
    0
    Folks, thank you for the insight!

    I would agree that severe discharge is a not common for starting batteries. Deep cycle batteries, on the other hand, are recharged after a deep discharge on a regular basis. So, a deep cycle battery can pull 2C for not an insignificant amount of time (seen that myself).

    40Ah battery cranking for 10 seconds at 300A discharges only 2%. I would agree that a slightly discharged battery would pull less than 0.3C. The battery is almost fully charged, so the charge current is not the initial charge current. Probably, this is why the primitive charging circuit in cars doesn’t age the battery too quickly. I’ve checked charging voltages in 2000s cars. 14.5V in BMW, 14.6V in Subaru.

    That's a useful curve. Where did you find it? I’m looking for in-depth references on lead-acid batteries.

    Manufacturers of deep cycle batteries almost always prescribe the initial charge current of no more than 0.3C. I haven’t seen data which would show how much and how quickly a battery would degrade if 0.3C is exceeded. On the other hand, several web pages say that lead-acids can tolerate abuse, but I haven’t yet seen quantitative data for that.

    My main interest is in deep cycle batteries.

    Cheers,
    - Nick
     
  6. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15
    "My main interest is in deep cycle batteries."

    Cheers,
    - Nick


    As is mine, for about 20 years now. If you really want to know deep cycle batteries "experiment". Build a lex-an box, get some powerful power supp lyes, a dozen batteries, new and used and dead, and have fun by trying everything you can think of.

    NOTE, be safe, use safety glass's and a face shield, use fire and smoke alarms in the experiment area.

    Try some of these sites

    http://www.mpoweruk.com/chemistries.htm

    http://www.cameronsoftware.com/ev/EV_BatteryPhysics.html

    http://www.mpoweruk.com/testing.htm#loadtest

    http://www.batteryfaq.org./

    http://www.madkatz.com/ev/batteryPOD.html

    http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery Voltages

    This should give you about an hour of reading.
     
    Last edited: Nov 20, 2011
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The most important link was missing:

    Battery University

    Everything you wanted to know about discharging, charging, and capacity of all types of battery chemistries and usage.

    --ETA: I've generally found that the higher current used to charge a deep cycle, the shorter the lifetime of the battery.

    For a fishing trolling motor used every weekend, a 2A-5A intelligent charger over the week should have it full by the next trip.
     
  8. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15
    Kender, here is the page that describes fast charging. What I get from testing batteries is that if the chemical reaction time will allow 300 amps to be drawn from the battery why can't we stuff it back in the battery almost as quick for bulk charge ???

    http://www.mpoweruk.com/chargers.htm

    "IUI Charging This is a recently developed charging profile used for fast charging standard flooded lead acid batteries from particular manufacturers. It is not suitable for all lead acid batteries. Initially the battery is charged at a constant (I) rate until the cell voltage reaches a preset value - normally a voltage near to that at which gassing occurs. This first part of the charging cycle is known as the bulk charge phase. When the preset voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another preset level. This second part of the cycle completes the normal charging of the battery at a slowly diminishing rate. Finally the charger switches again into the constant current mode (I) and the voltage continues to rise up to a new higher preset limit when the charger is switched off. This last phase is used to equalise the charge on the individual cells in the battery to maximise battery life. See Cell Balancing."

    DO NOT try this without engaging all the safety precautions. A spiral wound 12 volt battery has a dead short current of ~2000 amps and can be bulked charge at ~1000 amps (it takes very little time to move that much energy into such a small battery) I have done this many times to a set of batteries and they lived for ~4 years (very tolerant to abuse) and one is still living 10 years later. You never know how much a battery can do until you push it. Battery manufactures are very safe with there numbers for warranty reasons.
    Make sure to use a temp sensor on the battery post where the heat will show up first because the plastic case is very slow to transfer heat.
     
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