Series/parallel and Li-ion battery packs.

I've made two 4s battery pack for a hand held vacuum cleaner to replace the 12vSLA that it came with.

Two, because the internal circuit for charging the SLA isn't good for li-ion cells, so the idea is to have one pack charging extrernally whilst the other is in use and swap them.

Question: It would sometime be convenient -- like when cleaning the car interior -- to have an extended runtime.

I could easily add connectors to allow both packs to be connected in parallel; but they would be essentially unregulated one to the other.

What are the risks of doing so?

Anytime you connect batteries in parallel the one with higher charge passes current to the one with the lower charge, until both are equal - and this current could be VERY substantial depending on the relevant charges.

I'd add a high current schottky diode in series with each battery positive in, at the vacuum side of the connectors.

That way it can draw from both batteries once the voltage equalises, without risk of high circulating current from slightly different battery states.

NIce

rjenkinsgb said:

at the vacuum side of the connectors.

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I get the purpose of the diodes; but why at the vacuum side?

That would mean modifying -- physically -- the existing circuit board; or incorporating the diodes into the flying leads. The former would be difficult, the latter a bit iffy. Even if I mount it on a bit of veroboard and shrink wrap it; it smacks of things to come loose in the future.


Seems it would be physically easier to set it into the pack caps:

If that makes electronic sense?

I meant at the vacuum side electrically, not physically (not knowing the arrangements).

As you want two simultaneous batteries, that implies additional connectors; I meant fit the diode at the vacuum half of the connection, so the battery is not affected for other use.

If the batteries have separate charging connections, the diodes could be in the output leads of the batteries.

And remember the motor start-up surge current - I'd use something like a dual 20A TO-220 style diode with the two parts parallel (but I like to make things bombproof and as near idiot-proof as possible).
Running well down from the rated current should also mean less forward voltage drop so less heat dissipation.

rjenkinsgb said:

I meant fit the diode at the vacuum half of the connection, so the battery is not affected for other use.

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I'm trying to think of an alternative use that would be affectd by the presence of the diode? Or do you just mean extra power drawn?

rjenkinsgb said:

And remember the motor start-up surge current - I'd use something like a dual 20A TO-220 style diode with the two parts parallel (but I like to make things bombproof and as near idiot-proof as possible).

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I was looking at this https://www.mouser.co.uk/datasheet/2/308/1/SB1245_D-2320054.pdf. It states 12A continuous and 150 A surge.

The motor draws a little under 8A at 12.4 volts and <11A at 16.8V.

I'm not having a go at you -- the very last thing I would do here -- but what is the point of having/reading spec sheets if everyone ignores them and over rates every component?

If twin TO-220 is really required, then I'll just stick with swapping battery packs as needed. There is just enough space in the bay where the SLA lived to fit two of my 4S packs. Just.

Finding space for 4 x TO-220s would mean modifying the case or mounting them in some remote space like the handle, which is just too much bother.

If a diode were permanently installed in a two terminal battery system, it would be rather hard to charge it! Plus the extra drop on other loads.

Buk said:

The motor draws a little under 8A at 12.4 volts and <11A at 16.8V.

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That's under normal running conditions, once it has reached full speed.

Measure its DC resistance and work out the stall current! That's what it draws as it starts.
The 150A rating on the diode is for 8.3mA as an isolated surge. The motor takes rather longer than that to reach full speed, I suspect.

Buk said:

If twin TO-220 is really required, then I'll just stick with swapping battery packs as needed. There is just enough space in the bay where the SLA lived to fit two of my 4S packs. Just.

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One TO-220 package per battery; each has two diodes. If there is a high enough rated one, you could use one half per battery.
I was originally thinking of these:

>google<

Can you squeeze one of these in? That should do for both batteries.


If you are going to permanently install both packs, you could equalise them first to full charge and ensure all cells are balanced, then hard wire them in parallel at both the main terminals and all cell junctions. That may eliminate one balance/protection board and save space.

The diodes are only needed it you will be connecting packs that may be in different states of charge.

...

Things are different for mass-produced consumer stuff, but for industrial-grade gear that we work with as a company, or my own stuff, I try to design & build things in "brick outhouse" indestructable quality. There is a lot of our gear dating back to the early 80s still in continuous use, and customers keep coming back because they know our stuff is designed to last.

I found very early on that there is a massive difference between what should in theory be reliable and what actually is reliable in the real world.
As the famous quote, in some version or other: “In theory, theory and practice are the same. In practice, they are not.”

Turns out I have a pair of STPS3045CTs that I took out of an AT power supply.

And I've decided that 4S is probably too much for the 12V motor -- it works, but it gets very warm -- so I'm printing some caps to make pair of 3S packs.

In turn that frees up enough space to allow me to mount one (or both in parallel) of the TO-220s attached to a small heatsink in a convenient position.

Buk said:

I'm printing some caps to make pair of 3S packs.

Click to expand...

Turns out that's a bust. When the switch is thrown, the motor starts, but the initial inrush causes the voltage to drop to somewhere around 9V, and the circuit detects this, and assumes that the SLA it is expecting to be connected is flat and switches the motor off!

I see two options:

1) return to the 4S pack and limit the voltage.
2) Stick with the 3S and suppress the inrush current that causes the temporary, but dramatic voltage drop.

NTC thermistor?
In-line inductor?

The 12V lead acid battery would be considered flat at 11V, or possibly higher.
With three cells, that's 3.6V per cell even without adding any diode drop. You won't get full capacity range from the lithium cells, though that's not necessarily a bad thing, it will extend their life.

The simplest soft start would be a power resistor, shorted out by a transistor once the motor voltage gets above a certain point.

Four cells and a high current buck converter is another option; but that takes a lot of space; it would allow you to use more cells in series to increase the total capacity though.

I am surprised you are getting a lot of voltage drop? With good high current cells, there should be virtually no drop at massive currents.

I'm re-using the samsung 22Ps that came out of my Athlet. They've already had a long service life, but 5 out of the 7 seem to be in good condition, taking 85 to 90% of the rated capacity when charged. (The other two are down to just over 50%).

As for the voltage drop. I think it is only momentary at startup; but enough to trigger the SLA battery protection.

I was thinking of something like this. Costs a couple of dollars and takes no room.

(I'm also wondering if the gauge of the existing wiring from the battery to board could be too thin, causing or exaserbating the voltage drop. Trouble is, replacing it with something heavier is going to be a nightmare; they are tangled up with a mess of other wires that follow a toruous route through small cutouts between the two halves of the body. It's bloody difficult to get them back together without trapping wires somewhere. Adding thicker, stiffer wires would probably mean cutting new cutouts, and that would require planning.)

Buk said:

I was thinking of something like this. Costs a couple of dollars and takes no room.

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That should work, but it does need plenty of room around it - they get very hot.
That one says 190'C at full current; looking at the specs & curve chart I'd think it will be well above 100'C with the 10A load.

rjenkinsgb said:

I'd think it will be well above 100'C with the 10A load.

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Yeah. I'd have to be careful where I put it.

rjenkinsgb said:

The simplest soft start would be a power resistor, shorted out by a transistor once the motor voltage gets above a certain point.

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In this scenario, the transistor would be carrying the full load across its ▲V and discipating a fair amount of heat also?

With a decent power FET, the "on" resistance can be so low the power loss is trivial - eg.

1.8 milliohms, so I^2R at 10A = 180mW