Is it possible to mount a battery underneath the frame of a bike despite the lack of bolts? I want to use the bolts for a bottle. Does anyone sell strips of metal that bolt to the top and permit bolts below?

Look at SKS anyplace water bottle mounts (Amazon $14.95. What I would do is use them to mount your water bottle and use the mounts in the frame for your battery. SKS and others have attachments to mount a water bottle to your seatpost handlebars, anywhere almost. I even have a mount that attached to the rail of my seat (bicycle, of course).

That's pretty close - but I'm looking for something that's held in place by the bolts on the side with them. Also what is that corner bag on your bike?

The one in the picture is obviously a seat post mount, while the first one I referenced can be placed on the underside of the frame. The bag is just for tools.

Like 2old said the strap on SKS bottle mounts let you put a water bottle pretty much anywhere

https://www.amazon.com/SKS-Germany-1...37325585&psc=1

They obviously are not strong enough for a battery, especially one hanging upside down. If you want to mount a battery on the underside of your down tube you have a few options. If you have a steel frame you can drill and install rivnuts, put a bottle mount where you want it. A second option is radiator hose clamps to connect the battery mount where you want it. Since the battery will be hanging upside down in either case I would also put a velcro wrap around the battery.

No I mean what's the brand and make of the bag? I've never seen a corner bag that also goes to the side. It's less aero, but might be a convenient tradeoff for weight.
Aluminum, not steel. Is there something like a radiator hose clamp that mounts on the other side to the bolts that are there?

One of these might work

https://www.decathlon.com/products/u...QaAq-REALw_wcB

https://gearwest.com/topeak-versamou...IaAptrEALw_wcB

This one looks the strongest to me

https://dawntodusk.bike/product/bear-hug/

This silicone self fusing tape is excellent to wrap around the tube first to protect the paint. It alo crushes down making for a more secure mounting point. Put this on the tube then mount the external bottle mount on top of it

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Then once mounted I wrap one of these around the whole battery fixture. The silicone tape protects the paint where this mounts around back, plus adds some friction to keep things from slipping. I carry a couple of these in my touring panniers as well, excellent for securing things to the racks. You can get ones that are stretchy, but these solid ones mount very securely.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Rosewheel bag (Amazon AIR). Also, look at Luna's no drill battery mount kit (type recommended by Pop) or find something similar maybe using hose clamps and a Wolf Tooth system.

I'm not anywhere near converting yet (trying to research ahead first) but that looks pretty close to what I want.

DO you really want to put a battery under the bike when debris can bounce off the battery and cause a short, or maybe you hop a curb thats just a little bit too high??

Mount it in a protected area of the bike.

I'm generally not into hopping curbs. Also isn't the circuitry protected?

I highly suggest just using the battery on top with a strap, and maybe DIY'ing with some rivets a new water bottle holder. You can get a rivet gun and rivets on Amazon for not much, or your local LBS can probably but two in for you.

I have 2 DIY ebikes and had the same issue.

Why put the battery on top?

Main reason is that gravity works against the eyelets vertically, however when mounted on top of the tube, the majority of forces are lateral. Failure of an eyelet when mounted on top would lower the probability of a catastrophic failure that could eject the battery from the bike at high speed. Considering that the battery is the most expensive part of the system, usually, you really want to mitigate any of those possibilities.

Also in case of a crash, it is exposed and is not protected by the frame from impact below.

Excellent point about gravity; my daughter has been riding a similar system for a couple of years without difficulty even though she didn't put an extra strap around the battery as was suggested. The mount is an SKS usually employed to add a water bottle to a part of the frame without inserts.

Personally I care more about my bike than the conversion kit but whatever.

Many DIY battery packs are slung from top and down tubes using canvas bags and Velcro straps.
Test whatever pack you want to hang that it does not interfere with the rider.
This battery location is more susceptible to battery being knocked against walls and objects when parked out walking.
I prefer to put battery on back above wheel for protection or on bottle tube in a case.

Luna designed their wolf pup battery to mount under the down tube right next to the motor. It mounted with velcro straps, no eyelets, and was a pretty small battery. Seemed like a good spot for that battery.

I don't think there is a safe place on a bike in the event of a wreck. Seems like every bike wreck I've ever had spread debris over 20 square yard area. I think Luna's potted batteries have a real advantage in the event of a hard fall. Can't imagine hitting so hard the one would spill batteries.

I was simply referring to the battery portion of the electric kit you choose. Under the frame just is not an ideal location.

Cost of battery < cost of conversion kit
value of conversion kit < value of bike. generally, you like a bike and want to accelerate.

There's the Grin Tech Bottle Bob that uses a hose clamp, or Lekkie in New Zealand make a CNC machined mount that California Ebike is ordering in and should be available through their store website in a few weeks. However as others have said it's not enough to rely on to hold a battery upside down, EM3EV for example do not recommend mounting upside down their batteries that use the Hailong battery cradle.