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Rechargeable Batts — Parallax Forums

Rechargeable Batts

PureGrainPureGrain Posts: 21
edited 2006-11-24 16:23 in Robotics
When I read through the book and got to the servo setups, it recomended that I not use rechargeable batteries since they were 1.2v rather than 1.5v. Which kinda sucks because I purchased an additional 8 rechargable batteries so me and my kids could tinker with these guys and not worry about batteries as much.

Can these not be used at ALL? Are they going to mess up the servos for sure?

What do you recomend in place of these as far as rechargables? Can I get a RC type batt and charge kit to work with this?

Sorry for my ignorance. I just do not want to fry my chips nor my servos.

Comments

  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-09-03 02:06
    PureGrain -

    When you say "When I read through the book and got to the servo setups, it recomended that I not use rechargeable batteries since they were 1.2v rather than 1.5v." I'm not sure which book you're speaking about, nor what they intended to imply, but please consider this.

    Given some arbitrary target voltage, you can't use the same number of primary batteries (non-rechargables) in series as rechargables, since the base, individual cell voltages differ by .3 volts per cell. However, also consider this:

    4 x 1.5 = 6.0

    5 x 1.2 = 6.0

    So, the addition of one more rechargable battery in a batttery "pack", over what is found in a non-rechargable batttery "pack", results in the same final voltage. Therein, there is little difference, voltage-wise, between those two battery "packs". "Packs" here is intended to imply multiple batteries connectied in series, so that they appear as one voltage source.

    Regards,

    Bruce Bates

    Post Edited (Bruce Bates) : 9/3/2006 2:17:43 AM GMT
  • PureGrainPureGrain Posts: 21
    edited 2006-09-03 15:14
    Thanks Bruce for the comments. I understand that the voltage does not match up. I was not intending on mixing non-rechargable with rechargeable. I was just asking about using 4 x 1.2v rechargable batteries in the pack (deafualt 4 pack that came with the boe-bot). If what your saying is true above, then using the 4 x 1.2v will not provide enough voltage and could damage things?

    Thanks again for the help.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-09-03 15:46
    PureGrain said...
    then using the 4 x 1.2v will not provide enough voltage and could damage things?
    Nothing will be damaged.

    See, they just don't make battery holders that hold 5 cells (batteries.)· If you look around a little, you can find single-cell holders; so, with a "4" and a "1" you can make your own 5-cell arrangement.· There are 6-cell holders and, if you know what you are doing, you can jumper/short one of them (i.e. a cell slot) out and everything will be everything --·if a "6V" re-chargeable battery is your objective.
  • PureGrainPureGrain Posts: 21
    edited 2006-09-03 15:49
    I really do not have an objective. I just wanted to know why I could not use the rechargable batteries. 4 x 1.2 is fine with me for learning purposes. Thanks for all the help guys. ")
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-09-03 16:15
    PureGrain -

    I'm not sure how much better I can get my point across than what I've already said, but I'll try. Generally speaking, in any and all cases, NOTHING will be damaged, but performance (from a number of viewpoints) will unneccessarily suffer, when full voltage is not supplied to a given electrical or electronic device on the supply side (where the initial source of power exists).

    Here is what I offered before, and I'll now expand on it:

    Primary battery pack: 4 cells x 1.5 VDC = 6.0 VDC total

    Rechargable battery pack: 5 cells x 1.2 = 6.0 VDC total

    The above represent EQUIVALENT sources of power, presuming the mA (ampacity) ratings of the batteries·are equal, which they may or may not be.

    Now, try this on:

    Goal: Primary battery pack: 4 cells x 1.5 VDC = 6.0 VDC total

    A: Rechargable battery pack: 4 cells x 1.2 VDC = 4.8 VDC total

    B: Rechargable battery pack: 5 cells x 1.2 VDC = 6.0 VDC total

    Which of the above rechargable battery packs is MOST equivalent (from a voltage perspective) to·our "Goal" of 6.0 VDC?

    In what ways does A: above represent a poor and unaacceptable substitute?

    1. Initial voltage (4.8 VDC) far below the optimum voltage of (say) 6 VDC. This will yield poor performace in most cases, and overheating in some cases.

    2. Shorter period (duration) until cut-off voltage occurs. All active or dynamic electrical and electrical components generally have three voltages specified:

    Minimum Voltage (MinV), Typical Voltage (TypV), Maximum Voltage (MaxV)

    Example:

    Arbitrary and hypothetical motor specifications:

    MinV - 4.0 VDC, TypV = 6.0 VDC, MaxV = 7.5 VDC

    Explaination:

    As a battery is used, the operating voltage diminishes. With non-rechargables (non-alkaline) this voltage curve can be rather sharp. With alkalines, this voltage curve becomes less steep, and longevity over time (time to hit cut-off voltage) is increased. With rechargables, this curve is not only less steep, but is actually "propped up" a bit. (Ref: Battery performance curves for various battery chemistries for·additioanl information).

    Where you START (initial or starting voltage) very much determines how·SOON you will STOP (reach the cut-off voltage) of your electrical or electronic device. The longer time this takes, the better performance and longer duration you will get.

    Now reflect on the total voltage shown above for the various battery packs and configurations.

    Let's HOPE this didn't confuse you completely :-)

    Regards,

    Bruce Bates
  • PureGrainPureGrain Posts: 21
    edited 2006-09-03 16:48
    Nah, it did not confuse me the first time. I was just concerned if the lower voltage would mess anything up. In the book that came with my bot it claims NOT to use rechargable batteries because they will mess up the servos. I was just asking if it will mess them up for sure or if anyone has had trouble with them.

    your explanation actually answers other questions I had and was a great help. Thanks for your knowledge in this department.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-09-03 20:39
    What really sucks is that noone makes a 5 battery AA battery holder, you have to use a two and a three.

    They make singles, doubles, triples, quads, and sixes, though.

    Voltage is force, so generally lower voltage will not harm anything.
    The excessive force of over-voltage is what causes the damage as most devices are either designed to be or are self-protecting in a brown out state.

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  • Nickdawg940Nickdawg940 Posts: 7
    edited 2006-09-06 03:18
    I have been using 4 rechargeable batteries for atleast a month now and I've never even noticed a change.(Other than the money I saved [noparse];)[/noparse] )

    You shoudn't have any problems. Good Luck!
  • scary_botscary_bot Posts: 32
    edited 2006-09-06 08:02
    It all depends on what your running with it, and how much there draining from the batts. I ran four nihi cells, with no trouble untill I added a ping sensor. Then my run time dropped to about 1/4 of what I had. At that point I just built a five cell GP pack. Thay last pretty long. Even with the ·ping, servo's, Sharp IR's, and encoders. Of course most are pretty low drainers. The exception being the servo's, and the ping ....
  • dbc1218dbc1218 Posts: 33
    edited 2006-09-07 00:58
    i've been using 4 rechargeable duracell's on my boe bot for a while. Even though they are rated at 1.2v the four of them put out about 5.4v - 5.6v after a full charge. I don't know why this is but they work and thats all that matters.
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-09-07 06:28
    dbc1218 -

    Those almost have to be nickel cadmium (ni-cad) batteries based on the voltage you are reporting. The full-charge single cell voltage should be 1.25 VDC, NOT 1.20 VDC.

    Regards,

    Bruce Bates

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  • Bill PlanoBill Plano Posts: 14
    edited 2006-09-09 03:10
    I have used a 7.2v RC car battery pack by changing the connector to fit the VDC connector 6-9vdc.· Radio Shack has a 6v/1500mAh Ni-Cd rechargeable battery pack, 29-338 and a 7.2v/2000mAh battery pack/charger combo, 23-322.· This is no different than using a wall-wart to power up the BOE and using its built in regulator.

    Bill
  • Bill PlanoBill Plano Posts: 14
    edited 2006-09-09 03:12
    These should be available through your local store, or through radioshack.com.
    Bill
  • PureGrainPureGrain Posts: 21
    edited 2006-09-09 14:20
    That is exactly what I wanted to know. I wanted to get some RC batt packs like that from Wal-Mart or RadioShack and use them instead.

    Thanks for all the help guys.
  • PureGrainPureGrain Posts: 21
    edited 2006-11-24 16:23
    I have been using the same set of batteries for months now programming and working through the book on these projects. The batteries have lasted the whole time. Thanks again for all the help when I needed it. Now I must find something more interesting to do with these bots.
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