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Can I use 2 20v batteries to power a 40v tool?

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40v batteries for these 40v Dewalt tools are expensive, and I already have plenty of 20v batteries. Is it theoretically possible to build an adapter and use 2 20v batteries in place of a 40v battery?

If you ask here, that means you don't have the required knowledge to do it safely. So pay the price for the ad hoc battery.

So, while electrically, the below answers are correct, lithium ion wise they are not. Sophisticated Li-ion batteries, like the dewalt ones have electronic circuitry in them called a governor, or a BMS (battery management system). These devices monitor voltage/amp/temp/etc and have shut down circuitry in case of a fault. If you were to try building an adapter that could actually safely hold the batteries, figure out the dewalt pin configurations and actually wire the batteries in series so it outputted and mated with a connector that fit onto the 40volt tool correctly and securely, odds are the dewalt batteries or tool would sense something is amiss and go into protection mode. Only dewalt, with the ability to interface with the circuitry could ever make such an adapter. BTW it would be fairly large and cumbersome even if they did and probably throw the balance and weight off on the tool. Remember the default amperage for the 40v battery is 6aH, so series sum in the 20volt does nothing to the amperage, so two 20v 3 ah batteries is still 3ah@40v. You would need two 20volt flex 6ah to match the same AH, how heavy would that be!

Yes but do so carefully cause any mistake can result on the equipment frying or worst so pay attention. Like everyone mentioned before me in series two 20 volts would give you 40 volts.
How much amperage do you need and does one battery provide this amperage?
When placing the batteries in series the voltage adds up. When placing them parallel the Amperage is the one that gets added. If you are short on both I recommend paralleling two sets of batteries in series to accomplish this. It is important that you have enough current to power the device or you will have problems that can damage the device.

Yes it is theoretically possible as if you put two 20v batteries in series then the voltage gets added per battery so you will get your 40 volts. Now you would need to make an adapter that would connect the batteries in series which would be the hard part. I would make it so you can take the batteries apart and charge them as they are intended and "clipped together" in series to be put into your tool of choice. Hope this helps!

TL;DR: Yes but it may be dangerous.

In a word, yes. There are caveats, however.

A battery is just a series of cells. A "20V" Lithium battery is a series of 5 lithium cells. A 40V battery would simply be 10 cells in series, equivalent to two packs.

If you are going to build a series pack of two 20V batteries, they need to be as similar as possible (i.e. the cells are the same). Best way to do this is to buy two, new and only operate them in series. If they have a different charge/discharge history, they will wear poorly. If one pack or the other is more worn than the other, or has a lower total energy capacity, then it will discharge first. You must take special care not to continue using it, as this will reverse-charge the pack and cause serious damage, i.e. one or more cells may melt-down and breach, releasing toxic gases and a jet of flame. To recharge the batteries, the best practice would be to use a charger designed for 40V packs. The alternative, recharging 20V packs individually, would possibly place the packs in different states of charge, increasing the risk of reverse-charging as noted above.

One way to circumvent some of the requirements above (identical cells, identical states) would be to individually monitor the packs' voltages, using something like a simple, stand-alone voltmeter. If/when the individual pack voltage(s) drop(s) down to the recommended minimum (about 2.5V per cell, or 15V pack), they should be both recharged.

Naturally, all of this immediately voids warranty on all the equipment involved. Both batteries and your tool will be technically unsupported if anything fails. Also, there is a measure of danger involved.

As you see, doing it safely would involve getting a 40V charger and adapting it to your combo-pack (plus, purchasing new 20V batteries) or getting the electronics to monitor the individual batteries and doing so fervently. Experience with battery pack building and electrical engineering would be good here. It is important to be aware of the technical requrements and risks here.

I would recommend sticking with your 20V tools or purchasing the 40V tool(s) with the accompanying battery pack. Maybe you could find a buyer for the 20V tool collection and upgrade your entire tool set.

(note: the voltmeter link I provided is just an example of something small that would fit the bill; I do not vouch for its accuracy or endorse it.)

Hola, si conectas ambas baterias en serie puede que funcione. Debes tambien chequear la corriente que manejarás.
Saludos desde Uruguay.

I'd say it should be possible, theoretically. But you have to make sure the 40V devices don't draw more current than the 20V ones are rated for. Intuitively I'd guess they draw less, because double the voltage means the same power can be produced at half the current, but in case the 40V packs have been created for particularly heavy-duty devices (as in: a different class of tools than the 20V devices, e.g. 20V -> Electric Screwdriver, 40V -> Electric Chainsaw...) you still have to check.

It goes without saying that the 20V packs should ideally be joined without taking them apart, so you can charge them on their original charger, but it sounds like that was what you intended.

DISCLAIMER
I take no responsibility for anyone elses actions. The above comment is purely speculative as I do not have access to the batteries / tools myself. Check all the facts yourself. Someone else might have gotten it wrong ;)

Yes. In series voltage is additive and current is the same.
In parallel voltage is the same and current is additive.
To make a 40V 2A battery out of 2 20V 2A batteries, you connect in series.
To make a 20V 4A battery out of 2 20V 2A batteries, you connect in parallel.
So connect your batteries in series.

That's true. I wanted only to add that with 4x 20V batteries you can double the voltage as well as current. You make 2 package of 2x 20V batteries connected in series (you connect positive of one battery with negative of the other, the same for the other pack). Then you connect the two resulting packs you built in parallel (connect the two free negatives together and the two free positives together).
The packs double the voltage, the final connection doubles the current.

You can even do the opposite with the same result: make two pack in parallel to double the current and connect the two resulting packs in series to double the voltage.

Be careful to not short circuit (connect positive with negative) as the batteries can explode or make serious damages.

Agreed! I'm not so familiar with daisy chaining the batteries except well enough to know it provides a more even discharge and less damage to individual cells. The effect on power split I'm not so sure about!

there is a bigger issue with hooking up any battery sets... especially LiPo...
The cells need to be BALANCED... the LiPo's will need a sub circuit to ensure they are peeked individually..
the simply reason is that LiPo cells cannot be over discharged or they will likely self ignite... and or will never re-charge again.

Similarly, various NiCads, NiMh cells will also want to all be NEW, so that they all peak together when charging...
It is inadvisable to mix cells of different duty capability / charge rate / age / brand into one pack.
Simply buy a batch of new cells & build them into one pack from moment go, they will generally take 400 charges before going out of sync with each other.