I do not have access to an oscilloscope, so I ask here.

Short version: I charged a phone from a motorcycle battery. Phone battery broke.

Question: Can using a running vehicle's battery as a step-down regulator's input cause problems on the output?

Long version: I used this step-down buck regulator connected to a 12V motorcycle battery to charge a Samsung phone. Tested when vehicle was not running, output was calibrated to 5.01V. A cheap voltmeter was also connected the whole time, but only sporadically verified - all seemed in order.
Half an hour of driving later, phone was found to have turned off. Seconds after being turned back on, the battery started swelling. Phone still works.
Further confusing is the fact the phone has its own charging circuit which I'd expect to handle any instability AND the 2A output.

I am not claiming certainty that the setup caused the malfunction, but it seems likely.

[edit] The question, again, is whether the fact the input is from a running vehicle's battery - being used for the induction coil, etc, and charged at the same time, and whatever else might be important - is likely to affect the converter's output enough to produce the described effect.
[edit2] Connections are simple: From regulator, 2 cables to 12V battery, 2 cables to microUSB plug in phone. The regulator has capacitors on both sides, see the product photo.

  • 1
    \$\begingroup\$ A step down transformer works with AC in and out. Do you in fact mean a step down buck regulator and, if so, its design could be poor and cause serious problems. \$\endgroup\$
    – Andy aka
    Aug 11, 2016 at 8:11
  • \$\begingroup\$ I'm not sure from your description of the failure scenario whether the cell-phone itself is still working. E.g. with a known good & properly charged battery. \$\endgroup\$
    – FiddyOhm
    Aug 11, 2016 at 11:01
  • \$\begingroup\$ @fiddyohm Yes. So far 13hrs with no problems on new battery, but I haven't used the setup for charging again. \$\endgroup\$
    – kaay
    Aug 11, 2016 at 11:13
  • \$\begingroup\$ Did you connect directly to the battery? A downstream point might have more noise. Also, can you provide a schematic of your charger? \$\endgroup\$
    – stark
    Aug 11, 2016 at 11:41
  • \$\begingroup\$ @stark Almost directly on battery. Schematic? Motorcycle battery - this board - USB-A port - charger doctor - phone charging port - phone's internal charger - phone battery. rather straightforward. \$\endgroup\$
    – kaay
    Aug 11, 2016 at 11:55

2 Answers 2


Some phone charging ICs have marginal overvoltage tolerance. If your buck converter EVER exceeded the phone's spec you could cause problems. You need to be SURE both of the phone spec and the buck converter's behaviour.

Your subject line mentions regulator stability.
And, yes, some ICs in some circuits with some varying input conditions and some loads can behave somewhat differently than if input is lower voltage and steady. Ensuring it doesn't cause problems can be addressed by

  • Determining what a sensible Vsafe = V_phone_in_DC is

  • At least ensuring that requlator Vout does not EVER exceed Vsafe
    This may be achieved by a post regulator clamp or regulator or ...

  • Opionally and usefully testing the regulator under a representative range of Vin and load conditions to see if it may 'shake its head and yell loudly' occasionally when suitable perturbed.

Regulator stability may vary with surges, load dumps, dips, spikes, voltages at a higher level than when engine not running etc. Sometimes maybe phase of the moon, Murphy in a playful mood and bank holidays. For the latter a voltage clamp is a useful backstop.

The issue involves BOTH what the converter does and what the phone does about it. I would definitely not rely on "no more output volts than many USB ports I've charged from" (presumably = 5.1V as noted in question) as being 'safe'. I've seen USB ports at 5.5V AND phone regulator ICs that are rated at Vin = 5.5V ABS max. Really. That's insanity writ large, but happens.

I do not now recall the USB spec limits but if your phone will tolerate 4.5V I'd look at setting the voltage there.

The most important things here are probably the things that are as yet still missing. We know that one component on the PCB is an LM2596 buck regulator. We do not have a circuit diagram or any attempt at description of circuit operation. We as yet do not know what several more ICs on the board are. Peering at blown up amazon photos - a poor substitute for the guy wity the board just reading the part numbers off, where they are intact, we can see that the 8 pin soic by the output is a TPCA8016 - an N Channel 25A, 60 V MOSFET. I'd guess it was used to implement the output on/off function which we have not been told about.

Quite possibly pivotal is how the Vout_up and Vout_down buttons whose presence we have not been told about achieve their functionality.
Odds are they control up/down control pins on a digital pot IC. This MAY be the 4 pin (SOT89 3 pin + tab device) by the Vin connector block. If this is what it is then one needs to hope that it does not have flights of fancy when the Vin excerts (as auto supplies are wont to do) as if it returns semi-random values under duress Vout will be equally semi random. Not usually a good look.

Datasheets for two versions of the LM2596 IC are available here
TI version and ON Semi version.
These are both valuable for overlapping reasons.

One component which can have a very significant effect on regulator operation during either load or input transients is a "feed forward" capacitor across the upper resistor in the feedback divider network. This can be analysed formally in terms of frequency response but can also be understood as providing a path whereby fast output transients are coupled directly to the feedback pin rather than being divided by the feedback network and low pass filtered by the resistors plus stray and FB-pin capacitance. The effect is to cause the regulator to respond nearly instantaneously to fast transients with "fast" being determined by capacitor value and associated resistors.

In the ON Semi data sheet in diagrams in Fig 1 on page 2-top (2 examples), Fig15, Fig21, they show such a capacitor 'CFF' connected across the top resistor in the feedback network resistive divider.
If the Chinese buck board does NOT have such a capacitor present it should have - and one should be added if absent.
In the table on page 10 at bottom they say

    1. Feedforward Capacitor (CFF)
      It provides additional stability mainly for higher input voltages. For Cff selection use Table 1. The compensation capacitor between 0.6 nF and 40 nF is wired in parallel with the output voltage setting resistor R2, The capacitor type can be ceramic, plastic, etc..

See bottom of Table 1 at the top of page 11 for recommended values. They show something in the 5 nF - 20 nF range in their case. This to some extend (only) depends on the value of R1 (FB pin to ground) which can be 1K to 5K - they use 3K. What the Chinese use and if this is constant with digital pot variation depends on the circuit used - but if one end of the digipot resistor is grounded, which it more likely than not is, then R1 will vary with pot setting.

In the TI datasheet in fig35 page 26 an equivalent Cff capacitor is shown.

enter image description here

It is mentioned briefly at the top of page 16 where it misleadingly mentions use for Vout >= 10V.
It is discussed in (!!!) at the bottom of page 28 where a misleading comment is again made BUT table 6 shows values for use across the whole Vout range.

Is Cff present in the Chinese module as supplied?????????????????

The death of the cellphone battery suggests that substantial over-voltage was supplied. Use of a voltage clamp between regulator and cellphone seems liable to be a very very very good idea.


Note to fun editors.
Go. Away.
Defunned post will be rolled back or deleted en toto.

  • 1
    \$\begingroup\$ @kaay See existing answer for answer. You cannot tell what it does do for certain if it does bad things but you can see if it appears to do bad things and take steps to avoid them regardless. || Adumbrated / implied / copied from Answer: A regulator may produce spurious outputs under varying inputs. The sort of "noise" found in a vehicle can be severe. Trying to perturb the regulator while observing the output under various load conditions MAY show you if it is liable to misbehave. eg a noise spike may cause a buck regulator to miss a turn off point. Instead of "ringing the inductor it ... \$\endgroup\$
    – Russell McMahon
    Aug 13, 2016 at 13:05
  • 1
    \$\begingroup\$ ... IF you know Vmaxin for phone (which may be 5.5, 6. 8 , 10, 15v deep-ending how good the designer was, you can set a clamp accordingly. A zener is probably does not have a hard enough knee. A 5V1 zener may draw annoying current at 4V7 and not enough at 6V IF your phone cannot tolerate 6V. Knowing phone true spec helps. And zener curves. A Tl431 and MOSFET can give a nice sharp knee - eg from uA to A of clamp over 0.1V of regulator Vout increase. Speed is liable to be fast enough with \only minimal effort. || BUT .... \$\endgroup\$
    – Russell McMahon
    Aug 13, 2016 at 13:18
  • 1
    \$\begingroup\$ ... Let's simplify: YES buck converters can allow excessive Vout to pass when subject to the sort of noise experienced in a typical vehicle system. Converters that can tolerate vehicular electrical noise without giving bad Vout USUALLY can do so because they were designed to tolerate such. SOME SMPS ICs can help do this. I don't know what IC you are using and I don't know that the Chinese maker used a real one. | My dairly strenuous attempts to read the IC part numbers from the website have more or less failed - and you seem to prefer mystery to advising what you can re part numbers etc... \$\endgroup\$
    – Russell McMahon
    Aug 13, 2016 at 13:27
  • 1
    \$\begingroup\$ @kaay Please see extensions to answer. Happy to help if you help us do so. | What's the 4 lead IC / transistor by Vin connector. | Is a Cff present? | What is the phone's manual/spec-sheet stated Vinmax spec? | IF digipot helps add bad transient response a "normal" pot may help. | A Vout clamp is liable to be a not terrible idea. | The module's Vout turnoff function MAY be fast enough to protect against illegally rising Vout with quite simple circuitry. | Any other part number names that can be read? | ... \$\endgroup\$
    – Russell McMahon
    Aug 15, 2016 at 12:23
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    \$\begingroup\$ @kaay UsefulPeak Detector: A Classic Analog Circuit Still in Wide Use and some highly interesting and variably relevant stuff here \$\endgroup\$
    – Russell McMahon
    Aug 15, 2016 at 12:35

Lithium batteries have specific trend of charging, divided in minimum two phases (some chargers have up to four). First phase is charging battery with constant current until voltage level reaches nominal battery voltage. Second phase is keeping that voltage constant and charging current has exponential decline until current reach about 10% of rated battery's current. Using just constant voltage is not good enough for charging this type of batteries.

More about charging lithium batteries is provided here ( lithium ion battery charging).

  • 2
    \$\begingroup\$ the OP is charging the battery while its in the phone, so the phone takes care of the charging phases. \$\endgroup\$
    – Steve G
    Aug 11, 2016 at 8:30
  • 1
    \$\begingroup\$ CC/CV, I know. I just connected the regulator's 5V output to the phone's MicroUSB charging port. \$\endgroup\$
    – kaay
    Aug 11, 2016 at 9:30

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