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I am looking to replace an existing 12v Lead Acid battery configuration (two 7.5Ah batteries in parallel) for an old practice tennis ball machine. The machine consists of 12v dc motors and sensors to deliver overall speed, spin, height, etc., with some electronic circuitry to control the motors in tandem after receiving adjustments from analog dials.

  • Might a Lithium Ion battery be a viable replacement? The attractive qualities are the decreased weight and increased capacity.

  • My understanding is that most Lithium Ion batteries come in roughly 3.6v cell configurations which are then connected in series, generating 10.8v for a group of 3 cells. Some sellers call the resulting battery a "12v" and indicate the 10.8v is a "nominal charge". My impression is that the cells can also be slightly overcharged to exceed the 3.6v individual capacity. Is all this correct? Could a 10.8v Lithium Ion battery be used to replace a 12v dual 7.5Ah battery setup?

  • The dual 7.5 Ah setup powers the machine for approximately 3-4hrs; what would a good recommendation for that or, preferably, a longer period be?

  • Are there any potential drawbacks of foregoing Lead Acid and using Lithium Ion in this application?

  • Any recommendations on what would be a good lithium ion battery and charger configuration for this, or any other alternatives?

I really appreciate the help! Thanks!

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  • \$\begingroup\$ A fully-charged lithium cell has a voltage of about 4.2V (4.3V for some newer chemistries). \$\endgroup\$
    – Hearth
    Feb 5, 2019 at 0:20

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Not a good idea, unless you are planning to replace the charging circuitry as well.

Regardless of the circuitry being able to operate at a lower voltage, which is not guaranteed, lead acid batteries are very forgiving in how they are charged and handled tolerating relatively large voltage variations. Lithium batteries not so much, and a fire is a very real possibility.

A battery has a fixed voltage range given its chemistry, these are not electrically “tunable” in any way.

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  • \$\begingroup\$ Thanks Edgar, I should have mentioned that it is an external battery configuration and would be externally charged on a new dedicated charger. \$\endgroup\$
    – Hello
    Feb 6, 2019 at 18:51
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Each 12V battery offers 90Wh when new so the total is 180Wh at 12V.

Each Li Ion cell offers roughly 2.6Ah * 3.6V=9.36Wh so you need about 18 to 20 cells to get at least 180Wh.

if you use 3S8P =18 cells you only get 168Wh and 3.7 down to 3.0V x3 with is less voltage than a dead SLA so 4S5P or 20 cells is the minimum required to meet or exceed the original. 20x 9.36Wh=181Wh at an average voltage of 3.3V*4S=13.2V

You also need a balanced charger with a different profile for CC,CV and cutoff.

So a better idea is a deep cell Marine battery and car float charger if you tradeoff weight with additional capacity.

Ball speed is proportional to motor voltage.

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  • \$\begingroup\$ Thanks Sunny! The battery I'm looking at is rated 10.80 v, 26.1Ah, 282 Wh; my understanding is that for many lithium ion batteries, such as for power tools, they may be called 10.8v or 12v but are in fact interchangeable and have the same voltage profiles. The two motors on the machine controlling ball speed are both independently adjustable by speed gauges, and I've never had to use them at their top speeds. \$\endgroup\$
    – Hello
    Feb 6, 2019 at 19:17
  • \$\begingroup\$ That’s ok but top speed will be reduced as Vbat is reduced. But at least you have more Wh now \$\endgroup\$
    – Tony Stewart EE75
    Feb 7, 2019 at 0:44

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