What size converter charger should I get?

[tasslehawf]

Our vintage travel trailer doesn't currently have one, but it came with a marine battery that's rated at 85 Amp/hours. The smallest converter chargers I'm seeing are 35 amp. Is this small enough?

[Sam_Spade]

mrkoje wrote:

What I understand that you are saying is that by taking a 12v battery and connecting it in parallel with another 12v battery that because of differences in the two (even if they are the same battery model) will only produce approximately 1.5x capacity instead of 2x that would be expected?

I tried to explain it once. Let's try again.

Intitially you will get very close to 2X the capacity but as the batteries age, their relative capacity will change and the total will creep down.....because of what I stated before.
How long it takes for that to happen probably depends on luck more than anything.

I don't know of any better way to explain it. If the two produce different voltages, they will be "fighting" each other.

[mrkoje]

Matt_Colie wrote:

Sam Spade wrote:

mrkoje wrote:

Matt_Colie wrote:
Please do not parallel 12V jars. Even if they were side by side on the assembly line, by the end of the third week, they will not like playing in parallel. If you are lucky, you end up with capacity * 1.7. It will more likely be 1.5.

Please explain ...

Please search a bit.

Gross oversimplification summary:
If the two are not perfectly matched, part of the capacity of the stronger one will be expended trying to charge the weaker one. And when charging, the weaker one will sap off more of the charge.
And if one should fail completely, disaster can occur.

The disaster is rare and I think his 1.7 estimate is closer until they get really old but 6 Volts in series really are better.

Sam,
Thanks for picking this up before I got back here. Yes, that may be a gross oversimplification, but if we explained it in more detail many eyes would glaze over. There are so many important variables, that it is difficult to get arms around, but please believe us.

I have always loved a line from the original Stargate. Colonel Jack Anderson (Richard Anderson) was listening as the tech-savey lady explained how something worked. At the end, he looked at her and said,"That was a complete waste of a perfectly good explanation."

What I understand that you are saying is that by taking a 12v battery and connecting it in parallel with another 12v battery that because of differences in the two (even if they are the same battery model) will only produce approximately 1.5x capacity instead of 2x that would be expected?

Just for quick arguments sake - wouldn't it really depend at which rate you are using the batteries to determine how much usable capacity that would be usable?

Are you referring to a known scientific model here or an observed effect because of the low quality controls on typical 12v batteries say found at Walmart in which the rated capacities vs actual capacity could be substantially different from battery to battery?

I've tried researching a bit on what you are explaining and unfortunantley I have not come across anything that supports this claim yet. Im not saying I don't believe you at all. I just want to see the science of what you are saying.

Can you post a link to what you are referring to? My eyes don't glaze over when it comes to stuff like this.

[Matt_Colie]

Sam Spade wrote:

Matt_Colie wrote:
They said that you really can't charge a battery too hard if you do not over voltage the charge.

This is true but it's a funny way to state it.
It's kind of like stating that water will not overflow out of a bucket if you don't put too much IN it. 😉

Again, Thanks Sam,
The way I usually explain it is to say that charging batteries is like siphoning water.
If there is a lot of difference in levels, the flow can be fast.
When the container getting filled gets close to the source level, the flow will slow. If the containers safe level is the same as the source level, there can be no over fill.

I like your explanation better.

Matt

[Matt_Colie]

Sam Spade wrote:

mrkoje wrote:

Matt_Colie wrote:
Please do not parallel 12V jars. Even if they were side by side on the assembly line, by the end of the third week, they will not like playing in parallel. If you are lucky, you end up with capacity * 1.7. It will more likely be 1.5.

Please explain ...

Please search a bit.

Gross oversimplification summary:
If the two are not perfectly matched, part of the capacity of the stronger one will be expended trying to charge the weaker one. And when charging, the weaker one will sap off more of the charge.
And if one should fail completely, disaster can occur.

The disaster is rare and I think his 1.7 estimate is closer until they get really old but 6 Volts in series really are better.

Sam,
Thanks for picking this up before I got back here. Yes, that may be a gross oversimplification, but if we explained it in more detail many eyes would glaze over. There are so many important variables, that it is difficult to get arms around, but please believe us.

I have always loved a line from the original Stargate. Colonel Jack Anderson (Richard Anderson) was listening as the tech-savey lady explained how something worked. At the end, he looked at her and said,"That was a complete waste of a perfectly good explanation."

[wa8yxm]

Well, if Xantrex knwos what they are talking about you'd need about 25 amps for your converter, but yes, a 35 will be just fine.. Since your battery is likely a MARINE/deep cycle, the slightly faster charge won't bother it much.. That is a starting battery after all, designed to move a lot of current (relative to total amp horus) not a DEEP CYCLE which needs a slightly slower charge.

[Sam_Spade]

mrkoje wrote:

Matt_Colie wrote:



Please do not parallel 12V jars. Even if they were side by side on the assembly line, by the end of the third week, they will not like playing in parallel. If you are lucky, you end up with capacity * 1.7. It will more likely be 1.5.

Please explain ...

Please search a bit.

Gross oversimplification summary:
If the two are not perfectly matched, part of the capacity of the stronger one will be expended trying to charge the weaker one. And when charging, the weaker one will sap off more of the charge.
And if one should fail completely, disaster can occur.

The disaster is rare and I think his 1.7 estimate is closer until they get really old but 6 Volts in series really are better.

[Sam_Spade]

Matt_Colie wrote:

They said that you really can't charge a battery too hard if you do not over voltage the charge.

This is true but it's a funny way to state it.
It's kind of like stating that water will not overflow out of a bucket if you don't put too much IN it. 😉

[mrkoje]

Matt_Colie wrote:



Please do not parallel 12V jars. Even if they were side by side on the assembly line, by the end of the third week, they will not like playing in parallel. If you are lucky, you end up with capacity * 1.7. It will more likely be 1.5.

Please explain ...

[Matt_Colie]

Tassel,

I learned most of what I know about batteries from two guys that had run submarines in WWII in the pacific. They came back, so I guess they got it right.

They said that you really can't charge a battery too hard if you do not over voltage the charge. With a modern 12V jar, that is about 16V. None of the good converters go over about 14.5. Because if they do, then you have to replace a lot of light bulbs and other stuff.

The modern 3~4 stage chargers like a 9245, cannot hurt a battery. It will charge it at the best rate that the battery can take and then, after a few hours, it will back down and ask the battery how it is doing. That is what you want. If you later can go to a pair of GC2s, that same 9245 will serve you well.

Please do not parallel 12V jars. Even if they were side by side on the assembly line, by the end of the third week, they will not like playing in parallel. If you are lucky, you end up with capacity * 1.7. It will more likely be 1.5.

Before the depression killed my boat business (no, it is not coming back) I used to have this go-round with owners all the time. They didn't like it that I was right.

Matt

[Artum_Snowbird]

Progressive Dynamics 9245 is an extremely popular converter. That is a 45 amp unit that will work well with one or two batteries.

[RoyB]

Consider that a 12VDC battery will demand 17-20AMPS DC CURRENT if you hit them with 14.4VDC Boost Charge voltage whewn charging. This will give you a 90% charge state in a short three hour time frame and then you can reuse the battery again until it runs down to around 12.0VDC...

If you ever plan on going with two batteries on your trailer than you should get a 45AMP Converter/charger unit as both batteries will then demand a total of 34-40AMPS DC current when hit with 14.4VDC boost charge voltage....

You want your battery to make it through at least ONE NIGHT on a battery run and then if you have a 2KW Generator you can hook up your trailer shore power to it and get back in business again for the next day/night run off your battery...

ALot of places will camp at have generator run time restrictions so you want to get your batteries charged back up to their 90% charge state as soon as possible.

Roy Ken

[Sam_Spade]

2oldman wrote:
Is this a real consideration? I mean, can a single small battery be harmed by say a, 100a charger?

Yes if it is a cheap single stage one.

Getting one that is an automatic two or three stage is probably more important than the total capacity.

If it automatically steps down as the charge goes up, you can get one with a larger initial capacity for faster charging.

[2oldman]

Is this a real consideration? I mean, can a single small battery be harmed by say a, 100a charger?

[time2roll]

There are smaller but 35 amp will serve you well.

What brand/model are you looking at? Mostly plugged in or will you be charging on generator power?