DC DC Converters in series: safe or not? - Page 1
Forum section: electronics / projects

author: hamx
author_rank: Newbie
timestamp: July 01, 2013, 02:36:19 pm
content: Hi all, long time blog follower, first time posting on the forum (and registering..).I'm having an issue on which I can't find a lot of information, so where else is a better place to ask than here?I've been experimenting with two different DC DC Converters in series: one regulates the voltage of a (sealed lead acid) battery and protects it against deep discharge (by shutting off the load of the converter..). The second (in series) is a straight of the market DC-fed laptop charger (Kensington). This last one feeds a laptop.Why 2 converters?Well, most manufacturers of laptops protect their hardware by adding an "ID pin" so you can't use a charger that is not licensed... As I'm not intending to mimic the ID pin myself, I've resorted to a mainstream supplier.What's the issue?Ever since I've started experimenting, I have the issue that my battery starts to swell up.I've never "seen" it happen: it was only after a couple of days I was informed the system was no longer working well. The load never exceeds the maximum specified current of 8A and I've used the same kind of batteries for months now on a single DC DC Converter setup (so similar to the situation above except without the store-bought laptop-charger)I'm afraid (and not experienced enough with DC DC Converters) that putting these two converters in series is causing some oscillations or something. I was hoping that someone on the forum can tell me:Have I been doing a naughty "no-no" by putting these two in seriesHow can I make sure the converters are NOT interfering so I can run another test (and wait for the batteries to swell up or not)Thanks in advance!

author: digsys
author_rank: Unknown
timestamp: July 01, 2013, 02:51:34 pm
content: Quote from: hamxDo you mean - the 1st one charges / maintains the battery? If so, what is it's source? Doesn't seem clear to me.

author: hamx
author_rank: Newbie
timestamp: July 01, 2013, 03:02:58 pm
content: Sorry if unclear:There is a battery charger (3 Step specifically for lead acid batteries) involved as well.. Didn't mention it, sorry. This one is directly connected to the battery.Then the converters:The first one outputs a nice 12V to the second converter and also shuts down after the battery is below 10 Volts (to prevent the deep discharge) (this last function is the only reason I keep it in)The second converter outputs 19V and the correct ID-pin data to a Dell laptop.Hoping I cleared up something...

author: c4757p
author_rank: Super Contributor
timestamp: July 01, 2013, 03:10:48 pm
content: Quote from: hamx on July 01, 2013, 03:02:58 pmTake it out and replace it with a load switch that shuts the whole thing down at 10V. There's no reason to use another converter if you don't need conversion - you'll incur a nice little efficiency penalty as well.But absolutely, yes, two converters in a row can confuse each other's control loops, especially if one is poorly designed.

author: WBB
author_rank: Regular Contributor
timestamp: July 01, 2013, 03:29:03 pm
content: The battery charger could be the problem. A 3 stage charger can get stuck in stage 2 (boost mode) when charging a battery that also has a load connected. Some chargers allow you to disable stage 2 for that purpose. I do believe most chargers have a time limit on stage 2 by default, but that limit is likely too long to actually protect the battery.

author: hamx
author_rank: Newbie
timestamp: July 01, 2013, 06:55:47 pm
content: Quote from: c4757p on July 01, 2013, 03:10:48 pmI'd have to check out if there is something on the market that could replace the first converter. (I need something that is CE approved)Quote from: WBB on July 01, 2013, 03:29:03 pmDidn't think about it that it could get stuck in the second phase.. After checking indeed: there's a timer (4h 30min). I'm going to do a test to put a load on the charger and the battery, forcing it to get stuck. If that's the cause, I'd have to find a way to prevent this while keeping the load on...

author: WBB
author_rank: Regular Contributor
timestamp: July 01, 2013, 09:55:08 pm
content: The charger is almost certainly getting stuck. An additional problem is your setpoint of 10v to protect the battery from over discharge. Both problems are bad for the battery and the 2 combined can't be good.

author: hamx
author_rank: Newbie
timestamp: July 02, 2013, 06:11:54 am
content: Quote from: WBB on July 01, 2013, 09:55:08 pmHuh?This I don't get: why is it bad for the battery to limit its lowest voltage to 10V? I've been using them like this in a single converter setup for months now, without any problems!

author: vk6zgo
author_rank: Super Contributor
timestamp: July 02, 2013, 09:49:58 am
content: Quote from: hamx on July 01, 2013, 02:36:19 pmNO!! they are not "in series"---the output of one feeds the input of the other.The correct term which seems to have been lost in recent times is "in cascade"."In series" implies a different situation,such as when you want to increase the voltage available from a pair of DC power supplies by connecting the negative terminal of one to the positive terminal of the other,so that the two outputs add.(of course,this only works if both negatives are not connected to a common point.)

author: hamx
author_rank: Newbie
timestamp: July 02, 2013, 11:40:43 am
content: Quote from: vk6zgo on July 02, 2013, 09:49:58 amTrue, I stand corrected! I'll watch my language some more in the future...

author: WBB
author_rank: Regular Contributor
timestamp: July 02, 2013, 11:48:14 am
content: Quote from: hamx on July 02, 2013, 06:11:54 amLead Acid batteries do not like being drained completely flat. Assuming you're using a 12v battery, 10v is pretty much empty. Draining to 0% significantly shortens their life span.

author: hamx
author_rank: Newbie
timestamp: July 02, 2013, 11:57:53 am
content: Quote from: WBB on July 02, 2013, 11:48:14 amThe manufacturer specified about 9.2V as empty but safe. So I've put it 1V higher.I've seen these batteries after a number of very deep discharges (< 8V) and I really don't want to see that anymore!

author: WBB
author_rank: Regular Contributor
timestamp: July 02, 2013, 12:02:23 pm
content: Nice batteries. Maybe you can get the charging problem sorted and everything will be good.